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BIOLOGICA!. OCEANOGRAPHY
ANINTRODUCTION

SECOND EDmON

CAROL M. LALLI & TIMOTHY R. PARSONS

TtreOpen

University

SET BOOK

BIOLOGICA!. OCEANOGRAPHY

AN INTRODUCTION

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BIOLOGICA!. OCEANOGRAPHY

AN INTRODUCTION

Second Edition

CAROL M. LALLI

and

TIMOTHY R. PARSONS

University of British Columbia, Vancouver, Cañada

ELSEVIER

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British Library Cataloguing in Publication Data

Lalli, Carol M.

Biological oceanography: an introduction. - 2nd ed.

1. Marine biology 2. Marine ecology

I. Title II. Parsons, Timothy R. (Timothy Richard), 1932—

574.97

Library of Congress Cataloguing in Publication Data
Lalli, Carol M.

Biologocial oceanography: an introduction / Carol M. Lalli and
Timothy R. Parsons. - 2nd ed.
p. cm

Ineludes bibliographical references and Índex.

1. Marine biology 2. Marine ecology 3. Oceanography.

I. Parsons, Timothy Richard, 1932- II. Title

QH91.L35 96-42139

574.92-dc20 CIP

ISBN 0 7506 3384 0

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CONTENTS

ABOUT THIS VOLUME xi

CHAPTER 1 INTRODUCTION

1.1 SPECIAL PROPERTIES AFFECTING UFE IN THE SEA 1

1.2 CLASSIFICATIONS OF MARINE ENVIRONMENTS AND MARINE ORGANISMS 2

1.3 BASIC ECOLOGICAL TERMS AND CONCEPTS 4

1.3.1 r- and /C-selection 5

1.4 THE HISTORICA!. DEVELOPMENT OF BIOLOGICA!. OCEANOGRAPHY 7

1.5 SUMMARY OF CHAPTER 1 13

CHAPTER 2 THE ABIOTIC ENVIRONMENT

2.1 SOLAR RADIATION 16

2.1.1 Radiation at the sea surface 17

2.1.2 Radiation in the sea 18

2.2 TEMPERATURE 21

2.2.1 Sea surface temperatures 21

2.2.2 Vertical temperature distribution 24

2.3 SALINITY 25

2.3.1 Range and distribution of salinity 26

2.3.2 Biological importance of salinity 28

2.4 DENSITY 30

2.5 PRESSURE 33

2.6 SURFACE CURRENTS 34

2.6.1 Biological significance of currents 35

2.7 SUMMARY OF CHAPTER 2 36

CHAPTER 3 PHYTOPLANKTON AND PRIMARY PRODUCTION

3.1 SYSTEMATIC TREATMENT 40

3.1.1 Diatoms 40

3.1.2 Dinoflagellates 42

3.1.3 Other phytoplankton 45

3.2 PHOTOSYNTHESIS AND PRIMARY PRODUCTION 46

3.2.1 Methods of measuring biomass and primary productivity 48

3.3 RADIATION AND PHOTOSYNTHESIS 50

3.4 THE EFFECT OF NUTRIENTS ON GROWTH RATE 53

3.5 PHYSICAL CONTROLS OF PRIMARY PRODUCTION 58

3.5.1 Oceanic gyres and rings 60

3.5.2

Continental convergence and divergence

3.5.3

Planetary frontal systems

3.5.4

Shelf-break fronts

3.5.5

River-plume fronts

3.5.6

Island mass effect and Langmulr frontal zones

3.6

GLOBAL PHYTOPLANKTON PRODUCTIVITY

3.7

SUMMARY OF CHAPTER 3

CHAPTER 4

ZOOPLANKTON

4.1

COLLECTION METHODS

4.2

HOLOPLANKTON: SYSTEMATICS AND BIOLOGY

4.3

MEROPLANKTON

4.4

VERTICAL DISTRIBUTION

4.5

DIEL VERTICAL MIGRATION

4.6

SEASONAL VERTICAL MIGRATIONS

4.7

ZOOGEOGRAPHY OF THE HOLOPLANKTON

100

4.7.1

Patchiness

103

4.8

LONG-TERM CHANGES IN ZOOPLANKTON COMMUNITY STRUCTURE

108

4.9

SUMMARY OF CHAPTER 4

109

CHAPTER 5

ENERGY FLOW AND MINERAL CYCLING

5.1

FOOD CHAINS AND ENERGY TRANSFER

112

5.2

FOOD WEBS

117

5.2.1

The microbial loop

121

5.3

MEASURING SECONDARY PRODUCTION

123

5.3.1

Fleld studies

123

5.3.2

Experimental blologlcal oceanography

125

5.4

A COMPARISON OF MARINE AND TERRESTRIAL PRODUCTION OF

ORGANIC MATERIAL

134

5.5

MINERAL CYCLES

136

5.5.1

Nitrogen

137

5.5.2

Carbón

141

5.6

SUMMARY OF CHAPTER 5

143

CHAPTER 6

NEKTON AND FISHERIES OCEANOGRAPHY

6.1

NEKTONIC CRUSTACEA

147

6.2

NEKTONIC CEPHALOPODS

148

6.3

MARINE REPTILES

149

6.4

MARINE MAMMALS

149

vii

6.5 SEABIRDS 153

6.6 MARINE FISH 156

6.6.1 Fish migrations 162

6.7 FISHERIES AND FISHERIES OCEANOGRAPHY 162

6.7.1 World fish catch and fisheries management 163

6.7.2 Fluctuations in the abundance of fish stocks 164

6.7.3 Regulation of recruitment and growth in fish 167

6.7.4 Fishing and the use of near real-time oceanographic data 170

6.8 MARICULTURE 172

6.9 SUMMARY OF CHAPTER 6 174

CHAPTER 7 BENTHOS

7.1 BENTHIC PLAÑÍS 178

7.1.1 Measurements of benthic primary production 180

7.2 BENTHIC ANIMALS 180

7.2.1 Systematics and biology 182

7.2.2 Sampling and production measurements 191

7.3 DETERMINANTS OF BENTHIC COMMUNITY STRUCTURE 192

7.4 SUMMARY OF CHAPTER 7 193

CHAPTER 8 BENTHIC COMMUNITIES

8.1 INTERTIDAL ENVIRONMENTS 196

8.1.1 Tides 197

8.1.2 Environmental conditions and adaptations of intertidal organisms 197

8.2 ROCKY INTERTIDAL SHORES 198

8.2.1 Zonation 198

8.2.2 Trophic relations and the role of grazing and predation in determining

community structure 201

8.3 KELP FORESTS 202

8.4 SAND BEACHES 205

8.4.1 Environmental characteristics 205

8.4.2 Species composition 206

8.5 ESTUARIES 209

8.6 CORAL REEFS 213

8.6.1 Distribution and limiting factors 213

8.6.2 Coral structure 214

8.6.3 Diversity 215

8.6.4 Nutrition and production in reefs 217

8.6.5 Production estimates 218

8.6.6 Formation and growth of reefs 219

8.6.7 Zonation patterns on reefs 221

8.7 MANGROVE SWAMPS 222

8.7.1 What are mangroves? 223

8.7.2 Ecological features of mangrove swamps 224

8.7.3 Importance and uses of mangroves 225

8.8 DEEP-SEA ECOLOGY 226

8.8.1 Faunal composition 227

8.8.2 Species diversity 231

8.8.3 Biomass 232

8.8.4 Food sources 233

8.8.5 Rafes of biological processes 236

8.8.6 Future prospects 238

8.9 HYDROTHERMAL VENÍS AND COLO SEEPS 238

8.9.1 Chemosynthetic production 239

8.9.2 Vent fauna 239

8.9.3 Shallow vents and coid seeps 241

8.9.4 Unique environmental features of sulphide communities 242

8.10 SUMMARY OF CHAPTER 8 243

CHAPTER 9 HUMAN IMPACTS ON MARINE BIOTA

9.1 FISHERIES IMPACTS 248

9.2 MARINE POUUTANTS 251

9.2.1 Petroleum hydrocarbons 252

9.2.2 Plastics 252

9.2.3 Pesticides and other biologically active organic compounds 253

9.2.4 Heavy metáis 254

9.2.5 Sewage 255

9.2.6 Radioactive wastes 255

9.2.7 Thermal effluents 256

9.3 INTRODUCTIONS AND TRANSFERS OF MARINE ORGANISMS 256

9.4 IMPACTS ON SPECIFIC MARINE ENVIRONMENTS 258

9.4.1 Estuaries 258

9.4.2 Mangrove swamps 259

9.4.3 Coral reefs 260

9.5 SUMMARY OF CHAPTER 9 263

ARPENDIX t GEOLOGIC TIME SCALE 266

APPENOIX 2 CONVERSIONS 267

SUGGESTED FURTHER READING 269

ANSWERS AND COMMENTS TO QUESTIONS 270

GLOSSARY 287

ACKNOWLEDGEMENTS 304

INDEX 307

This Page Intentionally Left Blank

ABOUT THIS VOLUME

This volume is complementary to the Open University Series on
oceanography. It is designed so that it can be read on its own, like any other
textbook, or studied as part of S330 Oceanography , a third level course for
Open University students. The Science of oceanography as a whole is
multidisciplinary. However, different aspects fall naturally within the scope
of one or other of the major ‘traditionaF disciplines.Thus, you will get the
most of this volume if you have some previous experience of studying
biology. The other volumes in the Open University Series lie more within the
fields of physics, geology or chemistry (and their associated sub-branches).

Chapter 1 begins by describing unique properties that affect life in the sea,
and by making comparisons with life on land. The major categories used to
define marine environments and marine organisms are introduced, and basic
ecological terms and concepts that are central to studies of biológica!
oceanography are reviewed. The last section of Chapter 1 outlines the
historical development of this scientific discipline.

Chapter 2 considers some physical and Chemical features of the oceans
including light, temperature, salinity, density, and pressure, all of which
greatly influence the conditions under which marine organisms live. Major
water current patterns are described because they transport many marine
organisms, as well as dissolved gases and other Chemical substances, and
thus they affect the distributions of species and the size of populations in
particular areas.

Chapter 3 introduces the various types of phytoplankton, and describes the
way these floating plants manufacture energy-rich organic compounds by the
process of photosynthesis. Plants require energy from solar radiation, and the
effects of diminishing light levels with increasing depth are considered in
detail. Essential nutrients like nitrate and phosphate are present in relatively
low amounts in the lighted surface waters of the ocean, and the
consequences of varying nutrient concentrations on plant growth are
explored. Simple mathematical expressions are used to describe the
relationships between plant growth and light intensity or nutrient
concentration. Finally, vertical water movements that cause geographic
differences in biological productivity are examined.

Chapter 4 describes the major types of zooplankton and their general life
history patterns and feeding mechanisms. Vertical distribution of these
animáis is considered in relation to environmental differences with depth.
Many animáis migrate vertically in the water column., either daily or
seasonally, and the consequences of these migrations are considered. Broad
geographic patterns of distribution are also described, as well as smaller
scale patterns established by a variety of physical and biological influences.
Lastly, there is a discussion of long-term (decadal) changes in the abundance
and species composition of zooplankton communities.

Chapter 5 explores the flow of energy through marine food chains and food
webs, and explains the importance of using these ecological concepts to
predict yields of fish based on measurements of plant growth in particular
geographic areas. Different experimental approaches are also described that
can be used to study biological oceanographic issues concemed with the
dynamics of marine food chains. Finally, the important Chemical changes

that occur during the recycling of minerals are considerad, with particular
emphasis on nitrogen and carbón cycles.

Chapter 6 begins by describing the various types of nekton including the
larger crustaceans, squid, marine reptiles, marine mammals, seabirds, and
fish. The ecological importance of these larger animáis is emphasized, as
well as the consequences of their exploitation. The second half of this
chapter looks at fisheries management problems and at the causes of
fluctuation in the abundance of fish stocks.

Chapter 7 describes the major types of plants and animáis that live on the
seafloor, concluding with a section that explains how these organisms are
sampled, and how their rate of growth (or production) can be determined.

Chapter 8 reviews the environmental conditions and general ecology of
different bottom communities ranging from températe intertidal communities,
to tropical coral reefs and mangrove swamps, to deep-sea assemblages.

Chapter 9 considers the many ways in which human populations cause
changes in marine ecosystems. Attention is given to problems of overfishing
and exploitation of marine resources, and to the impacts of various types of
pollution. There is also consideration of the changes caused by the accidental
or delibérate introduction of marine species into new environments. Specific
impacts on estuaries, mangrove swamps, and coral reefs are discussed.

You will find questions designed to help you to develop arguments and/or
test your own understanding as your read, with answers provided at the back
of this volume. Important technical terms are printed in bold type where
they are first introduced or defined.

CHAPTER 1

INTRODUCTION

The oceans occupy about 71% of the Earth’s surface. The deepest parts of
the seafloor are almost 11 000 m from the sea surface, and the average depth
of the oceans is about 3800 m. The total volume of the marine environment
(about 1370 x 10 6 km 3 ) provides approximately 300 times more space for
life than that provided by land and freshwater combined. The ñame given to
our planet, ‘Earth\ is a synonym for dry land, but it is a misnomer in that it
does not describe the dominant feature of the planet — which is a vast
expanse of blue water.

The age of Earth is thought to be about 4600 million years. The ocean and
atmosphere formed as the planet cooled, some time between 4400 and 3500
million years ago, the latter date marking the appearance of the first forms of
life (see the Geologic Time Scale, Appendix 1). The earliest organisms are
believed to have originated in the ancient oceans, many millions of years
before any forms of life appeared on dry land. All known phyla (both extinct
and extant) originated in the sea, although some later migrated into
freshwater or terrestrial environments. Today there are more phyla of animáis
in the oceans than in freshwater or on land, but the majority of all described
animal species are non-marine. The difference in the number of species is
believed to be due largely to the greater variety of habitats on land.

1.1 SPECIAL PROPERTIES AFFECTING UFE
IN THE SEA

Why should life have arisen in the sea* and nol on land?

Marine and terrestrial environments provide very different physical
conditions for life. Seawater has a much higher density than air, and
consequently there is a major difference in the way gravity affects organisms
living in seawater and those living in air. Whereas terrestrial plants and
animáis generally require large proportions of skeletal material (e.g. tree
trunks, bones) to hold themselves erect or to move against the forcé of
gravity, marine species are buoyed up by water and do not store large
amounts of energy in skeletal material. The majority of marine plants are
microscopic, floating species; many marine animáis are invertebrates without
massive skeletons; and fish have small bones. Floating and swimming
require little energy expenditure compared with walking or flying through
air. Overcoming the effects of gravity has been energetically expensive for
terrestrial animáis, and perhaps it should not be surprising that the first forms
of life and all phyla evolved where the buoyancy of the environment
permitted greater energy conservation.

Two other features of the ocean are especially conducive to life. Water is a
fundamental constituent of all living organisms, and it is cióse to being a
universal solvent with the ability to dissolve more substances than any other
liquid. Whereas water can be in short supply on land and thus limiting to
life, this is obviously not the case in the marine environment. Secondly, the
temperature of the oceans does not vary as drastically as it does in air.

On the other hand, certain properties of the sea are less favourable for life
than conditions on land. Plant growth in the sea is limited by light because
only about 50% of the total solar radiation actually penetrates the sea
surface, and much of this disappears rapidly with depth. Marine plants can
grow only within the sunlit surface región, which extends down to a few
metres in turbid water or, at the most, to several hundred metres depth in
clear water. The vast majority of the marine environment is in perpetual
darkness, yet most animal life in the sea depends either directly or indirectly
on plant production near the sea surface. Marine plant growth is also limited
by the availability of essential nutrients, such as nitrates and phosphates, that
are present in very small quantities in seawater compared with
concentrations in soil. On land, nutrients required by plants are generated
nearby from the decaying remains of earlier generations of plants. In the sea,
much decaying matter sinks to depths below the surface zone of plant
production, and nutrients released from this material can only be returned to
the sunlit area by physical movements of water.

The greatest environmental fluctuations occur at or near the sea surface,
where interactions with the atmosphere result in an exchange of gases,
produce variations in temperature and salinity, and create water turbulence
from winds. Deeper in the water column, conditions become more constant.
Vertical gradients in environmental parameters are predominant features of
the oceans, and these establish depth zones with different types of living
conditions. Not only does light diminish with depth, but temperature
decreases to a constant valué of 2-4°C, and food becomes increasingly
scarce. On the other hand, hydrostatic pressure increases with depth, and
nutrients become more concentrated. Because of the depth-related changes in
environmental conditions, many marine animáis tend to be restricted to
distinctive vertical zones. On a horizontal scale, geographic barriers within
the water column are set by physical and Chemical differences in seawater.

Much of this text deais with descriptions of marine communities and the
interactions between physical, Chemical and biological properties that
determine the nature of these associations. Some attention is also given to
the exploitation of marine, biological resources. Despite the fact that the
oceans occupy almost three-quarters of the Earth’s surface, only 2% of the
present total human food consumption comes from marine species. However,
this is an important nutritional source because it represents about 20% of the
high-quality animal protein consumed in the human diet. Although a greater
total amount of organic matter is produced annually in the ocean than on
land, the economic utilization of the marine production is much less
effective. One branch of biological oceanography, fisheries oceanography, is
a rapidly developing field that addresses the issue of fish production in
the sea.

1.2 CLASSIFICATIONS OF MARINE ENVIRONMENTS
AND MARINE ORGANISMS

The world’s oceans can be subdivided into a number of marine
environments (Figure 1.1). The most basic división separates the pelagic and
benthic realms. The pelagic environment (pelagic meaning ‘open sea’) is
that of the water column, from the surface to the greatest depths. The
benthic environment (benthic meaning ‘bottom’) encompasses the seafloor

supralittoral high

water | 0 w water

PELAGIC ENVIRONMENTS
(plankton + nekton)

BENTHIC ENVIRONMENTS
(benthos)

hadal-

pelagic

lepipelagie

koQ m

Lmesopelagic

^1000 m

2000-3000m

►bathypelagic

.v 4000 m

^abyssopelagic

6000 m

10 000 m

Oceanic

Figure 1.1 The basic ecological divisions of
the ocean. The neritic (or inshore) petagic zone
is separated from the oceanic (or offshore)
pelagic zone by the edge of the continental
sheíf, whích ís generally at about 200 m depth.
Benthíc habitats are in bold type; pelagic
divisions are in blue. (Not to scale.)

and ineludes such areas as shores, littoral or intertidal areas, coral reefs, and
the deep seabed.

Another basic división separates the vast open ocean, the oceanic
environment, from the inshore neritic zone. This división is based on depth
and distance from land, and the separation is conventionally made at the
200 m depth limit which generally marks the edge of the continental shelf
(Figure 1.1). In some areas like the west coast of South America where the
shelf is very narrow, the neritic zone will extend only a very slight distance
from shore. In other areas (e.g. off the north-east coast of the United States),
the neritic zone may extend several hundred kilometres from land. Overall,
continental shelves underlie about 8% of the total ocean, an area equal to
about that of Europe and South America combined.

Further divisions of the pelagic and benthic environments can be made
which divide them into distinctive ecological zones based on depth and/or
bottom topography. These will be considered in later chapters.

Marine organisms can be classified according to which of the marine
environments they inhabit. Thus there are oceanic species and neritic
species depending upon whether the organisms are found in offshore or
Coastal waters, respectively. Similarly, plants or animáis that live in
association with the seafloor are collectively called benthos. The benthos
ineludes attached seaweeds, sessile animáis like sponges and barnacles, and
those animáis that crawl on or burrow into the substrate. Additional
subdivisions of the benthos are given in Chapter 7.

The pelagic environment supports two basic types of marine organisms. One
type comprises the plankton, or those organisms whose powers of

locomotion are such that they are incapable of making their way against a
current and thus are passively transported by currents in the sea. The word
plankton comes from the Greek planktos , meaning that which is passively
drifting or wandering. Depending upon whether a planktonic organism is a
plant or animal, a distinction is made between phytoplankton and
zooplankton. Although many planktonic species are of microscopic
dimensions, the term is not synonymous with small size as some of the
zooplankton inelude jellyfish of several metres in diameter. Ñor are all
plankton completely passive; most, including many of the phytoplankton, are
capable of swimming. The remaining inhabitants of the pelagic environment
form the nekton. These are free-swimming animáis that, in contrast to
plankton, are strong enough to swim against currents and are therefore
independent of water movements. The category of nekton ineludes fish,
squid, and marine mammals.

PLANKTON

FEMTO¬

PLANKTON

0 02-0 2 nm

PiCO-

PLANKTON

0 2-2 Onm

NAN0-
PLANKTON
2.0-20 n/m

MICRO-
PLAN KTON

20-200 nin

MESOPLANKTON

0.2-20 mm

MACRO-
PLANKT0N
2-20 cm

MEGA-
PLANKTON
20-200 cm

NEKTON

Centimetre

Nekton

2-20 cm

Decimetre

Nekton

2-20 dm

Metre

Nekton

2-20 m

VIRIO-

PLANKTON

8ACTERI0-

PLANKTON

MYC0-

PlANKTON

phyto¬

plankton

PR0T0Z00-

PLANKTON

METAZOO-

PLANKTON

NEKTON

Figure 1.2 A grade scale for the size classification of pelagic organisms.

Finally, it is often convenient to characterize pelagic organisms according to
size. Figure 1.2 presents one scheme for dividing plankton and nekton into
size categories. The categories encompass the smallest and most primitive
members of the pelagic environment, the viruses and bacteria, which are
placed in the femtoplankton (<0.2 /zm) and picoplankton (0.2-2.0 /zm)
categories, respectively. The largest nekton form the other extreme of the
size range, with species of whales that can be 20 m or more in length.
Phytoplankton and zooplankton (the latter including both unicellular
protozoans and multicellular metazoans) comprise the intermediate size
categories.

QUE5TI0N 1.1 In Figure L2, why is there overlap in size between the largest
plankton (macro- and megapJanklon) and the smallest nekton?

1.3 BASIC ECOLOGICA!. TERMS AND CONCEPTS

Basic ecological concepts are central to many studies of biological
oceanography, and certain ecological terms will be used throughout this text.
Marine organisms can be considered either individually or, more commonly
in ecological studies, on different collective levels. A species is defined as a

popularon [JenSftv

time

Figure 1.3 A iogistic growth curve. Note that
the population initially grows rapidly, then it
slows and eventually ceases to grow as it
reaches the carrying capacity of the
environment. K is the maximal population size
at carrying capacity, and r is the rate of
population growth.

distinctive group of interbreeding individuáis that is reproductively isolated
from other such groups. A population refers to a group of individuáis of
one species living in a particular place, and population density refers to the
number of individuáis per unit area (or per unit volume of water). The
various populations of micro-organisms, plants, and animáis that inhabit the
same physical area make up an ecological community.

The habitat of an organism is the place where it lives, but the term also may
refer to the place occupied by an entire community. The environment
consists of both nonliving abiotic (physical and Chemical) components like
temperature and nutrient concentrations, and biotic components that inelude
the other organisms and species with which an organism interaets (e.g.
predators, parasites, competitors, and mates).

The highest level of ecological integration is the ecosystem, which
encompasses one or more communities in a large geographic area and
ineludes the abiotic environment in which the organisms live. Examples of
ecosystems could inelude estuaries (see Section 8.5 for the component
communities of estuaries), or the total pelagic water column (with different
communities at different depths). Species diversity is often used to describe
the simplicity (or complexity) of communities and ecosystems; it can be
defined in several ways but, unless otherwise stated, the term is used
throughout this book to mean total number of species.

SI units (International System of Units) are now widely used in Sciences
since the system was adopted by the Conférence générale des poids et
mesures (CGPM) in 1960. However, much of the ecological literature
relevant to biológica! oceanography has used other types of units, and we
have continued to report results as they were originally published.
Conversions for more commonly used units can be found in Appendix 2.

1.3.1 r- AND ff-SELECTION

Each plant or animal species, whether pelagic or benthic, or marine or
terrestrial, has its own unique suite of biological and ecological features that
define its life history. Two very different types of life histories are
recognized as representing the extremes along a broad continuum of patterns.
One type is known as r-selected, and this life history pattern is
demonstrated by opportunistic species. The contrasting type of life history
is called K -selected, and organisms with this complement of characteristics
are referred to as equilibrium species. The terms r and K refer to different
portions of growth curves (Figure 1.3); r is the intrinsic rate at which a
population can increase, and K is the máximum population density that can
be supported by the environment. Populations that are kept at low densities
by abiotic or biotic environmental factors are influenced largely by the
parameter r, whereas populations that are at or near the carrying capacity of
the environment will be influenced by the parameter K.

Generally, r-selected species are of relatively small size, and they reach
sexual maturity early. They usually produce many young several times per
year. These opportunistic species typically live in variable or unpredictable
environments, and they are able to respond quickly to favourable conditions
or new habitats by rapid rates of colonization and reproduction. However,
r-selected species typically have little ability to compete with other species,
and they have high mortality. Consequently, populations of opportunistic
species tend to be short-lived. r-selection leads to high biological

productivity, as the organisms devote a large proportion of their available
energy to rapid growth and reproduction.

At the other extreme, fairly constant and/or predictable habitats favour
K-selected species that are larger in size and slow-growing, but long-lived.
They take longer to reach reproductive maturity and produce fewer young,
but death rates are fairly low. They are particularly adapted to live in areas
that are not subject to frequent disturbance, as they require sufficient time to
complete their life cycles. Equilibrium species tend to build up their
population sizes to the máximum that the environment will sustain (i.e. to
the carrying capacity of the environment), and they sustain this population
size for long periods by utilizing their resources very efficiently.

Table 1.1 summarizes other differences in these very different life histories.
As mentioned above, the concept of r- and Á'-selection can be applied to
pelagic as well as to benthic marine organisms, and it is also used in
terrestrial ecology. It should be noted that the different pattems are applied
in a relative sense to organisms. For example, although phytoplankton as a
whole have short life cycles compared to whales, within the group there are
both r-selected and /T-selected phytoplankton species. It should further be
stressed that the majority of organisms possess a mixture of r- and
tf-features. However, the importance of examining contrasting life styles lies
in recognition of the ways in which different species deal with competition,
predation, and environmental change. Throughout this book, you will be
encouraged to learn about the many different life history pattems of marine
plants and animáis, and to compare them on the basis of r- and K- strategies.

Table 1.1 A comparison of the life history pattems exhibited by r- and /T-selected
marine species.

r-selected

opportunistic species

A'-selected
equilibrium species

C1 i mate

variable/unpredictable

constant/predictable

Adult size

small

large

Growth rate

Time of sexual

rapid

slow

maturity

Reproduction

early

late

periods

many

few

Number of young

many

few

Dispersal ability

high

low

Population size

variable; usually

relatively constant;

below carrying

at or near carrying

capacity of
environment

capacity

Competitive

ability

low

high

Mortality

high; independent

lower; density

rate

of population
density

dependent

Life span

short (<1 yr)

long (>1 yr)

Pelagic/benthic ratio

high

low

1-4 THE HISTORICAL DEVELOPMENT OF BIOLOG1CAL
OCEANOGRAPHY

Human interest in the biology of the oceans can be traced back to
observations made in the fourth century B.C. by Aristotle, who described and
catalogued 180 species of marine animáis. The great sea-going expeditions
of the fifteenth and sixteenth centuries increased geographical knowledge of
the oceans and added incidental observations on biology, but modera studies
on the biology of the oceans did not really start until the middle of the
nineteenth century.

The British naturalist, Edward Forbes (1815-54) (Figure 1.4), is often
credited with being a founding father of oceanography, as he was one of the
first persons to conduct systematically designed studies of the marine biota.
He pioneered in the use of a dredge for obtaining samples of benthic marine
animáis, and he recognized that different species occupy different depth
zones. His book, The Natural History of the European Seas , was published
five years after his death, at the same time as Darwin’s The Origin of
Species. Unfortunately, Forbes is often remembered for the 1843 publication
of his azoic hypothesis, which claimed that marine organisms could not
exist at depths exceeding about 300 fathoms (550 m). Forbes was unaware
that there were already records of life from deeper areas of the sea. In 1818,
John Ross had obtained bottom samples containing worms and a starfish
from about 1920 m in Baffin Bay, west of Greenland. His nephew, James
Ross, led an expedition to the Antarctic in 1839-43 and collected benthic
animáis from as deep as 730 m. Such was Forbes’s influence, however, that
proponents of the azoic hypothesis clung to their beliefs despite increasing
contrary evidence. The idea that the coid, dark reaches of the ocean could
not possibly harbour any sort of life was finally refuted in 1860, when a
submarine cable was brought up for repair from more than 1830 m and was
found to have encrusting animáis growing on it. Now there was a growing
Ímpetus to organize deep-sea expeditions to study this vast unknown
environment and its inhabitants.

Charles Wyville Thomson became Edward Forbes’s successor as professor
of natural philosophy at the University of Edinburgh. In 1873, he published
one of the first texts of oceanography, The Depths of the Sea , based on a
review of early expeditions. Thomson also became organizer and leader of
the first oceanographic expedition to circumnavigate the world. This was the
Challenger Expedition of 1872-76, which travelled 110900 km visiting all
the major oceans except for the Arctic. The expedition was organized by The
Royal Society specifically to survey the oceans with respect to their physical
features, chemistry, and biology. The ship was a British naval sailing vessel
with auxiliary steam, specially outfitted for scientific work (Figures 1.5 and
1.6). Besides Thomson, two other naturalists completed the joumey.

Henry N. Moseley was described as an indefatigable scientist and an
enthusiastic amateur artist who contributed many drawings to the final
official report. John Murray, a Canadian-born Scotsman, carried out his
duties as a naturalist and later was instrumental in publishing the results of
Figure 1.4 Edward Forbes, a founding father the voyage. The contrasts between the scientists’ viewpoint of the voyage,
of modern oceanography. and that of the ship’s crew can be seen below.

Figure 1.5 H.M.S. Challenger.

‘Strange and beautiful things were brought to us from time to time,
which seemed to give us a glimpse of the edge of some unfamiliar
world.’ C. Wyville Thomson, The Challenger Expedition (1876).

From a naval officer’s diary: ‘Dredging was our bete noire. The
romance of deep-sea dredging or trawling in the Challenger , when
repeated several hundred times, was regarded from two points of view;
the one was the naval officer’s who had to stand for 10 or 12 hours at a
stretch, carrying on the work ... the other was the naturalist’s ... to
whom some new worm, coral, or echinoderm is a joy forever, who

Figure 1.6 The zoological laboratory on board
H.M.S. Challenger.

retires to a comfortable cabin to describe with enthusiasm this new
animal, which we, without much enthusiasm, and with much weariness
of spirit, to the rumbling tune of the donkey engine only, had dragged
up for him from the bottom of the sea.’

Although a vast amount of information about the ocean had already been
amassed by the time of the Challenger Expedition, much had been collected
incidentally or in bits and pieces by individual scientists. The Challenger
voyage attempted to intégrate biology, chemistry, geology, and physical
phenomena, and it established systematic data collection using standardized
methods. For these reasons. the Challenger Expedition is considered to mark
the beginning of modern oceanography. Over 76 scientists analysed the
collections made during the voyage, and it took 19 years before all 50
volumes of final reports were published under the direction and financial
patronage of John Murray. The expedition produced a basic map of the
seafloor, and proved without doubt the existence of life at great depths. The
biological samples yielded 715 new genera and 4417 new species of marine

Figure 1.7 Some of the 3508 new species of
radioiarians collected by the Challenger
Expedition, and described and illustrated by the
Germán zoologist Ernst Haeckel.

1 l AñCHlCAPSA , 3*íi HALICAPSA, 7 PLAimjfiSA,

8 ClATW ROBU RSA , íl JO ARCHIPEflA, 11 il ARCMISCÉNIUM , J3 CLADOSCENUÍM,
U-JJi PTEROSGENIUM, É71B ACRQCORÜMA. 1020 TCTRtóORETHHA

organisms; 3508 of them were new species of Radiolaria (a group of
protozoans) (Figure 1.7), all described by the great Germán biologist Ernst
Haeckel

The monograph on echinoderms was researched and complied by
Alexander Agassiz of the United States, who said T felt when I got
through that I never wanted to see another sea urchin and hoped they
would gradually become extinct..

The Challenger Expedition also discovered the true nature of Bathybius
haeckelii, described by Tilomas Huxley in 1868. Bathybius was a thin layer
of mucus-like jelly covering the surface of preserved mud samples, and
Huxley believed that it represented a primordial living slime which carpeted
the deep seafloor. This ‘organism’, however, proved to be a precipítate of
calcium sulphate, the result of mixing alcohol with seawater to preserve
bottom samples.

‘Never did an expedition cost so little and produce such momentous
results for human knowledge.’ — Ray Lancaster

Many other expeditions followed on the path blazed by the Challenger , and
a few of those that made major contributions to biological oceanography are
listed in Table 1.2. John Murray went on to organize the Michael Sars
expedition of 1910, and in 1912 he co-authored a classic text in general
oceanography, The Depths of the Ocean , with Johan Hjort, a Norwegian
scientist.

Table 1.2 Major biological oceanographic expeditions.

Vessel

Country

Dates

Major objectives/advances

Challenger

U.K.

1872-76

Global biological
collections; existence of life
in deepest waters

Blake

U.S.A.

1877-86

Dredging; Caribbean and
Gulf of México collections

Princesse Alice

I and II;

Hi ronde lie

I and II

Monaco

1886-1922

Deep-sea collections

Albatross

U.S.A.

1887-1925

Deep sea; Pacific and Indian
Ocean collections

National

Germany

1889

Plankton collections

Valdivia

Germany

1898-99

Vertical distribution of
pelagic organisms; deep-sea
biology

Michael Sars

Norway

1904-13

Mid- and deep-water
collections; North Atlantic

Dana I and II

Denmark

1921-36

Deep-water global
collections; fishery research

Discovery

I and II

U.K.

1925-39

Antarctic ecology

Meteor

Germany

1925-38

Atlantic biology

Galathea

Denmark

1950-52

Deep-sea dredging to

10000 m; global collections

Vitiaz

U.S.S.R.

1957-60

Biology of trenches

Trieste

(bathyscaphe)

Swiss/U.S.

1960

Deepest manned dive
(10916 m, Mariana Trench)

Alvin

(submersible)

U.S.A.

1977

Discovery of deep-sea hot
springs

Work was being conducted on marine plankton even before the time of the
Challenger . The first person reported to have studied marine plankton was a
surgeon and amateur naturalist, J. Vaughan Thompson, who towed a simple
fine-meshed net to collect plankton off the coast of Ireland in 1828. His
studies resulted in the first description of the planktonic stages of crabs.
Charles Darwin also used a similar net to collect marine plankton during
his stint as an unpaid, and seasick, naturalist on the voyage of the Beagle,
from 1831 to 1836. In 1847, Joseph Hooker recognized that the diatoms
collected in plankton nets were plants, and he suggested that they played the
same ecological role in the sea as green plants do on land. However, it was
not until 1887 that the term ‘plankton’ (see Section 1.2) was actually defined
by Víctor Hensen, a professor at Kiel University who also led the first
oceanographic expedition entirely devoted to quantitative collections of
plankton (the Germán ‘Plankton Expedition’ on board the National , see
Table 1.2) The word ‘plankton’ was more critically defined in 1890 by Ernst
Haeckel, and today the word encompasses all drifting organisms including
plants (phytoplankton), animáis (zooplankton), and bacteria
(bacterioplankton).

Monographs on many different groups of zooplankton were available by the
end of the nineteenth century, and taxonomic guides to phytoplankton were
beginning to appear. Increasing attention was also given to those organisms
that were too small to be collected by nets, as indicated below.

‘... H. H. Gran [a Norwegian scientist] now commenced using his big
steam centrifuge for centrifuging the water samples from different
depths, (and) he continued to avail himself of its help until the end of
the cruise. By means of it he was able to collect in a little drop below
the microscope all the most minute organisms, and in spite of the
movements of the little ship and the vibration from the propeller, he
was able with his microscope to study the many hitherto unknown
forms in their living State, to draw them, and to count the number of the
different species.’ — John Murray’s account of a cruise on Michael
Sars, as related in The Depths of the Ocean (1912).

The late 1800s and early 1900s also marked the establishment of several
marine and oceanographic laboratories, many of which were founded by
biologists. In Europe, the Germán zoologist, Antón Dohrn, established the
Stazione Zoológica de Napoli in 1872; the station was unique at that time in
that its facilities were available to visiting scientists of other countries. The
Marine Biological Association of the United Kingdom started a laboratory in
Plymouth, England, in 1888. In 1906, Prince Albert I of Monaco
established an oceanographic museum and aquafium to house extensive
collections made by his research ships (see Table 1.2). In America, Louis
Agassiz (father of Alexander Agassiz) established the first marine biological
laboratory on the east coast in 1873; this was later (1888) moved to Woods
Hole where it became the Marine Biological Laboratory. During this period,
Spencer Baird started the first of a series of laboratories devoted to fisheries
studies in Woods Hole and, in 1930, the Woods Hole Oceanographic
Institution was officially established. On the west coast of America, William
Ritter (a student of Alexander Agassiz) founded a research organization in
1905 that eventually (in 1924) became the Scripps Institution of
Oceanography in La Jolla, California. Now, almost all countries bordering
the ocean have oceanographic or fisheries stations.

The history of biological oceanography is as much intertwined with the
chemistry of seawater as it is with the nature of marine animáis and plants.

An understanding of the ecological role of phytoplankton required
measurements of nutrients in seawater, and these were first carried out by
the Germán chemists Brandt (1899) and Raben (1905). Later, the English
chemist H. W. Harvey extended the earlier work of measuring nitrates and
phosphates to inelude other nutrients such as iron and manganese.

The beginning of an ecological understanding of the sea carne from the first
textbooks that attempted to intégrate biological data with the physical and
Chemical properties of the sea. One of the earliest and most comprehensive
texts in this respect was The Oceans (1942) by Sverdrup, Johnson, and
Fleming. Later ecological works included the 1957 publication of Volume 1,
Ecology , of the Treatise on Marine Ecology and Paleoecology edited by
J. W. Hedgpeth; G. Riley’s Theory of Food Chain Relations in the Oceans
(1963); and the delightfully written and illustrated text by Alister Hardy,
titled The Open Sea: Its Natural History (1965).

Alister Hardy was a wonderful storyteller, equally at home describing
his hot-air ballooning experiences in England, or relating shipboard
experiences like the following from a voyage to Antarctic waters on
board Discovery I. ‘Expecting a host of surface life we slung a bo’sun’s
chair (a board supported by ropes on each side like a swing) cióse to
the water ... right in front of the bows themselves. Here Kemp and I
took turns with a hand-net and bucket. For sheer pleasure it was ideal:
swinging in mid-air and gently rising and falling with the swell over the
deep blue surface which occasionally rose to bathe and cool one’s legs;
one advanced like a gliding and soaring bird with nothing in front of
one but the virgin ocean, as yet quite undisturbed by the bows behind
... I rodé in triumph, fishing out treasure after treasure as they carne
floating towards me on the very gentle undulating swell. An experience
never to be forgotten/ From Great Waters (1967).

One unfortunate development in the history of biological oceanography was
that much of the study of the top marine predators, the fish, fell under a
sepárate discipline, that of fishery Sciences. This carne about because the
most abundant marine fish form the basis of commercial fisheries. The
branch of fishery Sciences was founded in about 1890, led by Alexander
Agassiz in the United States, Frank Buckland in England, and
W. C. Mclntosh in Scotland, all of whom looked to ocean Science as a
means of improving fish catches.

In 1902, the International Council for the Exploration of the Sea (ICES)
was established under the auspices of King Oscar II of Sweden. This
organization attempted to intégrate physical studies of the oceans with
biological investigations of fish, but this was not completely successful.
Scientists trained in physics or chemistry and those trained in biology used
different methods and approaches, and they proceeded to work independently
of each other. ICES proved unable to forcé effective legislation concerning
control of endangered stocks, or overfishing, and the organization was not as
innovative in developing new fishing techniques and discovering new
fisheries stocks as the fishermen themselves. After the war, ICES sponsored
co-operative expeditions in the North Sea and North Atlantic, paid for by the
national institutions in each participating country.

Fisheries management strategies tended to concéntrate on economic models
based on fish abundance and catch, while ignoring the rest of the biology of
the sea. Classic texts on fisheries dealt primarily with the effeets of

harvesting on fish population size (e.g. On the Dynamics of Exploited Fish
Populations by Beverton and Holt, 1957). Increasing human populations and
increased demand for food resources have driven commercial fisheries to
expand their fleets while developing new ways to lócate fish schools and to
harvest the stocks more efficiently. Diminishing stocks of exploited species
have alerted fisheries scientists to the appreciation that the abundance of fish
in the oceans is not only related to the numbers of fish removed, but is also
greatly influenced by ocean climate. A relatively new field known as
fisheries oceanography has developed which attempts to relate
oceanographic data to fluctuations in fish stocks.

Biological oceanography began as a descriptive Science, and basic
observations on the biology of marine organisms and their environments
continué to be an important aspect of biological oceanography. However, the
development of new techniques and apparatus has changed the scope and
scale of oceanographic research. Sonar, originally developed during World
War II to detect enemy submarines, was later employed to study the
topography of the seafloor, find fish schools, and, most recently, to lócate
and follow concentrations of larger zooplankton. Submarines and scuba
diving became more sophisticated, and now both are used to obtain in sita
observations of marine life. Underwater sound recording with hydrophones
is now employed to study communication in marine mammals and the
echo-location of prey by mammals and some fish. Computers have greatly
decreased the time needed to analyse data routinely, and they are useful
tools in simulating oceanographic events. The development of satellites and
remóte sensing has made it possible to map the ocean temperature and to
trace ocean currents. The scale of research in biological oceanography now
extends from laboratory studies on the effects of environmental change on
single phytoplankton cells, to employing satellite imagery to obtain global
patterns of plant production at the sea surface.

There is now an awareness that meteorological events in the atmosphere and
climatic changes in the ocean and on land are connected over vast distances,
and that humans can also produce impacts on the sea which can be measured
on a global scale. The latter inelude the air- and water-borne dispersal of
pesticides and other Chemicals, and the overexploitation of fish and marine
mammal stocks. As human impacts on the ocean increase, an expanding
base of knowledge about the ecology of the seas becomes essential to
address questions of exploitation and pollution with more certainty.

QUESTlüN 1,2 Why is the history of biological oceanography so recent
compared with the development of terrestrial biology?

1.5 SUMMARY OF CHAPTER 1

1 The marine environment provides about 300 times more inhabitable
space for living organisms than that provided by land and freshwater
combined. All known phyla of plants and animáis originated in the sea, and
there are presently more phyla represented in the oceans than on land.

2 In comparison to life in air, the fluid nature of the ocean provides a
buoyant environment in which the effects of gravity on living organisms are
reduced. Because of this, marine organisms do not have to invest energy in

building large proportions of skeletal material, and they expend
comparatively little energy in maintaining buoyancy and in locomotion.

3 Plant growth in the ocean is limited to the near-surface regions because
light does not penétrate very far in seawater, and it is further limited by the
low concentrations of essential nutrients (e.g. nitrate and phosphate) that are
present at these depths. Because almost all life in the sea depends directly or
indirectly on plants, the total plant production at the surface determines the
amount of animáis that can be produced.

4 Vertical gradients in environmental parameters (e.g. light, temperature,
pressure) establish depth ranges with distinctive environmental
characteristics.

5 Despite the vast extent of the marine environment, only 2% of the
human diet comes from marine resources. However, this represents 20% of
the high-quality animal protein consumed by humans.

6 The benthic environment encompasses the seafloor, and those species of
plants and animáis that live on or within the seabed form the benthos. The
pelagic environment is that of the water column, from the sea surface to the
waters immediately above the seafloor; inshore waters form the neritic zone,
and offshore waters form the oceanic región. Plankton and nekton inhabit the
pelagic environment; the distinction between the two groups of organisms is
based on relative swimming ability, with nektonic species being stronger
swimmers that are able to move independently of current direction.

7 Pelagic organisms can be classified into size categories ranging from
femtoplankton (viruses) through intermediate sizes to the largest nekton
(whales).

8 On an ecological scale, organisms can be considered individually or in
assemblages that inelude populations of a single species, or communities
made up of the populations of many interacting species. The highest unit of
ecological integration is the ecosystem, which encompasses one or more
communities as well as surrounding environment.

9 The life history patterns of all species form a continuum that ranges
between the extremes described by r-selection and Á'-selection.

Opportunistic species are adapted to live in variable or transitory
environments by having short life eyeles, production of many young, and
high dispersal ability; however, these r-selected species have high mortality
rates and their populations are often of short duration. Á^selected species
live in stable environments and usually have population densities near the
carrying capacity of the environment; these equilibrium species typically
have longer life spans, produce relatively few young, and have
comparatively low death rates.

10 Edward Forbes (1815-54) is regarded as the founding father of
biological oceanography, and the Challenger Expedition of 1872-76 marks
the beginning of systematic oceanographic studies that intégrate physical
phenomena, water chemistry, and biology.

11 New techniques developed in the mid- to late-1900s expanded the scope
and scale of oceanographic research. These inelude sonar, submarines, scuba
diving, underwater sound recording, and remóte sensing from satellites, all
of which are now used to investígate life in the sea.

Now try thefollowing questions to consolídate your understanding of this
Chapter.

QUESTIOM 1.3 When díd the firsl vertebrales appear in the ucean? Refer lo
ihe Geologic Time Scale in Appendix 1.

QUESTIOH 1.4 What are some eharacteiisuc features of the environmem ai
3(XX) m depth in the water column?

QUESTION 1.5 What is the greaiesi depth reaehed hy a matined diving vessel
in the oceans? Refer lo Table 1.2.

CHAPTER 2

THE ABIOTIC ENVIRONMENT

In order to understand the ecology of the seas, it is necessary to understand
the abiotic physical and Chemical constraints of the marine environment to
which the resident organisms are adapted. Some of these ecological
constraints derive from the nature of seawater itself, with special properties
related to the fluid nature of water and to the Chemicals dissolved in this
fluid. Other abiotic environmental features important to life in the sea result
from the interplay between the Earth’s atmosphere and the sea surface.

2.1 SOLAR RADIATION

Sunlight is as essential to life in the sea as it is to life on land. Some fraction
of the solar radiation penetrating into the sea is absorbed by plants during
photosynthesis, and this energy is used in the conversión of inorganic matter

Figure 2.1 A schematic illustration of the
passage of solar radiation through the
atmosphere and sea surface, and the proportion
of photosynthetically active radiation (PAR)
available at depth in the sea.

to organic compounds. Some wavelengths of light are absorbed by water
molecules and are converted to heat, which establishes the temperature
regime of the oceans. In addition, light in the sea Controls the máximum
depth distribution of plants and of some animáis. Vision in animáis is
dependent on light, and certain physiological rhythms such as migrations and
breeding periods may be set by periodic light changes.

2.1.1 RADIATION AT THE SEA SURFACE

Biological oceanographers have tended to use a variety of units to measure
solar radiation at the sea surface, and to measure light intensity at depth in
the sea. For that reason, conversions between different units are given in

Figure 2.2(a) Solar radiation spectra before
and after passage through the atmosphere,
showing the zone of PAR (photosynthetically
active radiation) or visible light.

(b) Percentage of light reflected from a calm
sea surface as a function of sun angle.

(c) Changes in overhead solar radiation received
at the sea surface with latitude and season in
the Northern Hemisphere (contour lines
represent latitude).

Appendix 2, and these should be referred to when necessary. Two light units
used for biological studies in the sea are the einstein (E), which measures
photons (one einstein is a mole of photons, or 6.02 x 10 23 photons), and the
watt (W), which measures the energy of radiation. The energy of radiation
depends on the wavelength of the light, but for photosynthetic radiation (400
to 700 nm), one W m -2 is approximately equal to 4.16 ¡xE m~ 2 s” 1 .

Solar radiation Corning from the Sun to the outside of the Earth’s atmosphere
is fairly constant (Figures 2.1 and 2.2a). About half of this energy is
absorbed and scattered in the various layers of the atmosphere, so that the
amount reaching the sea surface is about 50% of that received at the top of
the atmosphere. Some of this is reflected back into the atmosphere from the
sea surface (Figures 2.1 and 2.2b). The amount reflected depends on the Sun
angle and becomes very large below a Sun angle of 5 o to the horizon.

During any day, the actual amount of radiation reaching the sea surface at
any point is thus a function of the Sun angle, the length of the day, and
weather conditions. The Sun angle is determined by the time of year, time of
day, and by the latitude. At the Equator, radiation from an overhead Sun is
fairly constant throughout the year but, at 50° N, the seasonal variation in
incident radiation ranges from about 1000 fiE m -2 s -1 in January to over
4000 fiE rrT 2 s -1 in June (Figure 2.2c).

QUE SISON 2.1 What is the approximate máximum solar radiation received at
the surface oí the A rabian Sea oíf Bombay (latitude ca* 20 N) in
(a) Sepiember and (h) January? Refer to Figure 2,2c,

A summary of temporal variations in radiation at the sea surface is given in
relative units in Figure 2.3. The diel variation is the change in solar radiation
over 24 hours (i.e. the difference between day and night). Diurnal variations
are those that occur during hours of daylight due, for example, to cloud
cover. Seasonal variations are most marked at high latitudes. This is
particularly so within the Arctic Circle where there can be 24 hours of
sunlight on the ocean surface during the summer. Differences in the input of
surface solar radiation account for much of the difference in photosynthesis
by phytoplankton at discrete localities in the ocean.

2.1.2 RADIATION IN THE SEA

In comparison with other liquids, water is relatively transparent to solar
radiation, but much less so than air. Of the sunlight penetrating the sea
surface, about 50% is composed of wavelengths longer than about 780 nm.
This infrared radiation is quickly absorbed and converted to heat in the
upper few metres (Figure 2.1). Ultraviolet radiation (< 380 nm) forms only
a small fraction of the total radiation, and it also is usually rapidly scattered
and absorbed, except in the very clearest ocean waters (Figure 2.4). The
remaining 50% of the radiation comprises the visible spectrum, with
wavelengths of between approximately 400 and 700 nm that penétrate
deeper in the sea. These are of particular importance for animáis with visión,
and because they are also approximately the same wavelengths used by
plants in photosynthesis. These wavelengths are often referred to as
photosynthetically active radiation (PAR). The máximum intensity of PAR
with the Sun directly overhead is about 2000 ¡jlE m -2 s -1 . Obviously this
valué will vary with Sun angle, and it decreases to zero as the Sun
approaches the horizon.

Figure 2.3 Temporal variations in surface
solar radiation. (Relative scales).

PERIOD OF
VARIATION

Diel

Diurnal

Seasonal

(50°N)

Figure 2.4 The penetration of light of different
wavelengths into clear oceanic water. The lines
indícate the depths of penetration for 10% and
1% of the surface light levels.

wavelength (nm)

300 400 500 600 700

Latitudinal §

(equatorand 3

Arctic summer) «

24h

As light passes through water, it is both scattered and absorbed, with
different wavelengths of the visible spectrum penetrating to different depths
(Figure 2.4). Red light ( ca . 650 nm) is quickly absorbed, with only about
1% still remaining at 10 m in very clear seawater. Blue light (ca. 450 nm)
penetrates deepest, with about 1% remaining at 150 m in clear water.

There is an exponential decrease of light intensity with depth. An extinction
coefficient, k, is calculated to express this attenuation of light. The extinction
coefficient of seawater can be calculated from measurements taken with a
radiation meter lowered into the sea, and using the following equation in
which /o is the surface radiation and Id the radiation at depth:

k _ logf(/o) ~ log ^ /p) (2 1)

depth (m)

CUESTION 2.2 The radiación ai 10 m is 50^ of the surface radiación as
measured with a radiación meter. What is the extinctíon coefficient?

As you might have inferred from Figure 2.4, the extinction coefficient, k , is
different for various wavelengths of light. It is about 0.035 m -1 for blue
light, but about 0.140 m _1 for red light. However, if many partióles are
present in the water, the blue light is scattered more than the red, and this
will affect the colour spectrum of undersea light, resulting in a shift of the
most deeply-penetrating wavelength toward a green colour (Figure 2.4). The
extinction coefficient is also affected by the amount of coloured, dissolved,
organic material in seawater, and by the amount of chlorophyll contained in
living phytoplankton and in plant debris. In the clearest ocean water of the
tropics, light which is visually detectable by a deep-sea fish may penétrate to
more than 1000 m (Figure 2.5). In turbid Coastal waters, scattering and
absorption of light are increased by the presence of much silt and numerous
phytoplankton, and the same amount of light may not reach 20 m.

QUESTION 2,3 Novice scuha di vei s are ofien di sappoinled ai seeing a coral
reef for ihe firsi time beeause of the monotony of colour compared with
colour photographs and films of red's. Why is this so?

Three vertical ecological zones in the water column are defined by the
relative penetration of light in the sea (Figure 2.5). The shallowest zone is
called the euphotic zone, and it is defined as that región in which light is
sufficient to support the growth and reproduction of plants. Here, there is
sufficient light for plant production by photosynthesis to exceed the loss of
material that takes place through plant respiration (see Section 3.2). The

Figure 2.5 The vertical ecological zones
established by light penetration in the sea. Note
that the light intensity scale is logarithmic with
depth. The positions of the vertical broken lines
delimiting the three ecological light zones are
approximate only (see text).

light intensity (pWcm 2 )

amount of light required for photosynthetic production to just balance
respiratory losses in plants is known as the compensation light intensity.
The depth at which photosynthetic production is balanced by plant respiration
is called the compensation depth, and it defines the lower boundary of the
euphotic zone. Thus the euphotic zone extends from the surface to a depth of
just a few metres in turbid inshore regions, to a máximum depth of about
150 m in very clear, tropical oceanic water. In any región, the compensation
depth (De), and thus the depth of the euphotic zone, can be calculated from:

Dc = 'og,('o>- U> g,('c) (2 . 2)

Surface radiation (/o) is measured directly, and k is calculated from
equation 2.1 assuming a wavelength of 550 nm. The valué used for the
compensation light intensity (le) varíes with different species of
phytoplankton as well as with the previous history of light adaptation of any
particular species. For example, heavily shaded phytoplankton can adapt to
lower compensation light intensities. In general, however, valúes for le will
range between 1 and 10 (i E m~ 2 s -1 .

Below the euphotic zone is the dimly lighted disphotic zone, a región where
fish and some invertebrates can see, but where light is too low for positive
net photosynthesis (i.e. loss of plant material through respiration exceeds
plant production by photosynthesis over 24 hours). However, living
phytoplankton which have sunk from the euphotic zone may be present here.

The deepest and largest región in the open ocean is the dark aphotic zone;
this extends from below the disphotic zone to the seafloor. Here, sunlight
cannot be detected by any biological System. This vast región does not
support plant Ufe, and is spatially removed from the initial link in the marine
food chain.

QUESTION 2,4 What do yon ihink is the biológica] role of moonlighl (see
Figure 2.5) in the sea?

2.2 TEMPERATURE

Water temperature is one of the most important physical properties of the
marine environment as it exerts an influence on many physical, Chemical,
geochemical, and biological events. Temperature Controls the rates at which
Chemical reactions and biological processes (such as metabolism and growth)
take place. Temperature and salinity variations combine to determine the
density of seawater, which in turn greatly influences vertical water
movements with consequent changes in Chemical and biological events
within the water column. Water temperature partly determines the
concentration of dissolved gases in seawater; these inelude oxygen and
carbón dioxide, which are profoundly linked with biological processes.
Temperature is also one of the most important abiotic factors influencing the
distribution of marine species.

2.2.1 SEA SURFACE TEMPERATURES

There is a continuous exchange of heat and water between the ocean and
atmosphere. The seas are heated primarily by the infrared wavelengths of

(a)

Figure 2.6 The global distribution of sea surface temperatures (°C) in (a) February and
(b) August.

solar radiation. The radiant energy of these wavelengths is quickly
transformed into heat by absorption. The heating effect of sunlight is
confined to the immediate surface of the ocean, with 98% of the infrared
spectrum being absorbed within about the first metre of the water column.

Sea surface temperatures vary with latitude (Figure 2.6). Surface
temperatures can exceed 30°C in the tropical open ocean, and approach 40°C
in shallow tropical lagoons. At the other extreme, water surface temperatures
in polar regions may be as low as — 1.9°C the freezing point of typical
seawater. The modérate regime of surface seawater temperature is in sharp
contrast to air temperatures affecting terrestrial ecosystems; these range from
as high as 58°C (in northern Africa during summer) to —89°C in the
Antarctic during winter (Figure 2.7). The temperature regime of the oceans
is buffered by certain physical properties of water. Water has a very high
specific heat, meaning that it can absorb or lose large quantities of heat with
little change in temperature. Furthermore, the oceans are cooled primarily by
evaporation and, because the latent heat of evaporation for water is the
highest of all substances, great quantities of heat can be transferred and
stored in water vapour with relatively little change in water temperature.

It is sometimes convenient to designate biogeographic zones based on sea
surface temperatures. The following zones lie within the boundaries set by
the annual average surface temperatures given in the right column:

(b)

Tropical

Subtropical

Températe

Polar

25°C

15°C

5°C (northern limit)
2°C (Southern limit)
< 0-2° or 5°C

Températe zones in both hemispheres are characterized by a mixture of
subpolar and subtropical water, and by having the maximal annual
temperature range. Although attempts have been made to ascribe latitudinal
limits to these temperature zones, this has little or no ecological significance
in the pelagic environment where currents displace water of different
temperatures away from their origins, and where water temperature changes
very gradually through mixing. In pelagic communities, faunal boundaries
follow certain isotherms (lines of equal temperature), or are more precisely
described by combinations of temperature and salinity which define
distinctive bodies of water (see Section 2.4).

The mean daily variation in surface temperature in the open ocean is very
small, generally less than 0.3°C, and it is usually imperceptible at 10 m
depth. Even in shallow water, the daily surface temperature change is less
than 2 o C. Temperature changes o ver 24-hour periods are therefore of little
importance to plankton and fish, unlike residents of intertidal and terrestrial
ecosystems which can be subjected to very considerable differences between
day and nighttime temperatures.

Annual surface temperature fluctuations (compare Figures 2.6a and b) are
very small in Antarctic waters and are less than 2-5°C in Arctic and tropical

highest recordad aír
temperatura
(Libya, 1922)

average surface
temperature near
Equator

lower observed limit,
breeding range of
Emperor penguín
(-18 to -62)

°C

surface maxíma in
shallow Coastal
waters

average máximum
surface temperature,
Red Sea

average mínimum
22 "\ for survival of
reef coráis

-10

-60

-62

*1.33 average
Antarctic surface

1.65 mínimum
\ Antarctic bottom
*1.87 freezíng point,
at atmospheric
pressure

lo west recordad air
temperature
(Antárctica, 1983)

Figure 2.7 Temperature ranges in the sea
(blue) and on land (black).

seas. In températe and subtropical areas, they are large enough to influence
biological events significantly. In the open ocean at latitudes of 30-40°,
where clear skies permit the maximal heat gain in summer and maximal heat
loss in winter, the annual variation is about 6-7°C. However, the western
areas of the North Pacific and North Atlantic have annual variations of up to
18°C because of the prevailing westerly winds that bring very coid
continental air masses over these regions in winter and warm continental air
in summer. In shallow marginal seas, and Coastal areas generally, the
fluctuation in water temperature closely parallels air temperature, and annual
variations may exceed 10°C.

In addition to daily and seasonal variations in surface temperature, there are
longer term climatic changes that affect marine ecology. Some of these
events are deduced only from changes in seafloor sediments that suggest
dramatic temperature changes in the overlying water during the geological
past. Other climatic changes can be observed in the present time and inelude
major perturbations such as the El Niño events in the Pacific Ocean. These
are eyelieal changes in sea surface temperature occurring every two to ten
years that have widespread impaets on marine ecology as well as on global
weather. An El Niño can have a catastrophic impact on commercial fisheries
in affected areas, and the details of this event are considered in Section 6.7.2.

2.2.2 VERTICAL TEMPERATURE DISTRIBUTION

Turbulent mixing produced by winds and waves transfers heat downward
from the surface. In low and mid-latitudes, this creates a surface mixed
layer of water of almost uniform temperature which may be a few metres
deep to several hundred metres deep (Figure 2.8). Below this mixed layer, at
depths of 200-300 m in the open ocean, the temperature begins to decrease
rapidly down to about 1000 m. The water layer within which the
temperature gradient is steepest is known as the permanent thermocline.
The temperature difference through this layer may be as large as 20°C. The
permanent thermocline coincides with a change in water density between the
warmer low-density surface waters and the underlying coid dense bottom
waters. The región of rapid density change is known as the pyenoeline, and
it acts as a barrier to vertical water circulation; thus it also affeets the
vertical distribution of certain Chemicals which play a role in the biology of
the seas. The sharp gradients in temperature and density also may act as a
restriction to vertical movements of animáis.

Temperature decreases gradually below the permanent thermocline. The
thermal stratification of the oceans is shown schematically in Figure 2.9. ín
most oceanic areas, the water temperature at 2000-3000 m never rises above
4°C regardless of latitude. At greater depths, the temperature declines to
between about 0°C and 3°C. The temperature of deep water at the Equator is
within a few degrees of that of deep water in polar regions. The only
exceptions to coid deep conditions are found in certain localized areas of the
deep sea, where bottom water temperature may be elevated by geothermal
activity (see Section 8.9)

In températe climates, seasonal thermoclines (Figure 2.8) are established in
the surface layer during the summer. These result from increased solar
radiation that elevates surface temperature at a time when winds are lessened.
Thus there is little turbulent mixing to promote downward movement of
heat, and a thermal stratification is set up in the near-surface waters. This
phenomenon persists until autumn, when the surface water is cooled

temperature ( Ú C)

5 10 15 20

Figure 2.8 A generalized temperature protile
for températe latitudes. The solid line shows the
winter condition with a mixed surface layer of
homogeneous temperature overlying the
permanent thermocline. The dashed lines show
the formation of seasonal thermoclines that
develop in the surface water in spring and
summer due to elevated solar radiation and
warming coinciding with lessened wind.

and increasing winds cause sufficient turbulence to mix the upper layers and
break down the thermocline. Beeause permanent and seasonal thermoclines
greatly affect biological productivity on a global and temporal scale,
respectively, more will be said about thermal stratification in later sections.

Jo what way does se a water temperatura affect faunal distribuí i ons?

The physiological ability to cope with environmental temperatures plays a
large role in determining the distributional limits of marine organisms. The
majority of marine animáis (i.e. invertebrates and fish) are poikilothermic
species with a varying body temperature that approximately follows the
ambient water temperature, but marine mammals are homoiothermic and
maintain a constant body temperature. Animáis that can exist in
environments with a wide temperature range are known as eurythermic.
Such species tend to have wide distributional ranges or they live in regions
of considerable temperature fluctuation, such as températe intertidal zones.
Those species that are restricted to narrow temperature limits are called
stenothermic. They inelude such groups as reef-building coráis, which
require a minimum temperature of 20°C, as well as those species that are
restricted to coid waters. The geographic range of cold-stenothermic species
may be very wide; for example, some species that are found at shallow
depths in the Arctic are also present at depths of 2000-3000 m in Equatorial
areas where similar coid temperatures prevail.

2.3 SALINITY

Salinity refers to the salí contení of seawater. For our purposes, salinity can
be simply defined as the total weight (in grammes) of inorganic salís
dissolved in 1 kg of seawater. However, salinity is not measured by weight
beeause it is difficult and tedious to dry all the salts in seawater. Salinity is
more easily and routinely determined with a salinometer that measures
electrical conductivity, which increases with increasing salt contení. The
major elements are present in the form of ions, with sodium and chloride
predominating. The ten major constituents listed in Table 2.1 make up about
99.99% of all the dissolved substances in the ocean.

Figure 2.9 A generalized and schematic
cross-section, showing the main thermal layers
of the oceans and their average temperatures at
the Equator.

Equator

Why does thís list not inelude such comnion and biológica! I y importa ni
elements as oxygen, nitrogen, and i ron?

Other elements and compounds in the oceans are present at lower
concentrations than those listed in Table 2.1. Some of these, like oxygen and
carbón dioxide, exist as dissolved gases and will be considered later. Those
elements that are linked with biological processes (e.g. nitrogen in the form
of nitrate) exhibit highly variable concentrations, unlike the ions listed in the
table. Although there also are dissolved organic compounds in seawater, all
are in concentrations too low to affect salinity.

Table 2.1 The major constituents of seawater with a salinity of 35.

Ion

Concentration
(g kg -1 )

% by weight
of all salts
in the sea

Chloride (Cl~)

18.98

55.04

Sodium (Na + )

10.56

30.61

Sulphate (S0 4 2- )

2.65

7.68

Magnesium (Mg 2+ )

1.27

3.69

Calcium (Ca 2+ )

0.40

1.16

Potassium (K + )

0.38

1.10

Bicarbonate (HCO^)

0.14

0.41

Bromide (Br~)

0.07

0.19

Borate (mainly H3BO3)

0.03

0.07

Strontium (Sr 2+ )

0.01

0.04

Because the concentrations of most of the major constituents are not
significantly affected by biological and Chemical reactions, they are said to
show conservative behaviour. This property results in the constancy of
composition of seawater. That is, the total salinity may vary, but the relative
proportion of each major ion to the total remains stable, as do the ratios of
the concentrations of each major ion to the others. These ionic ratios depart
from normal only in localized regions such as estuaries, which receive an
inflow of freshwater containing different relative proportions of major ions.

2.3.1 RANGE AND DISTRIBUTION OF SALINITY

Variability in salinity is linked with global climate. Salinity in surface waters
is increased by the removal of water through evaporation, and it is decreased
primarily through the addition of freshwater via precipitation, either in the
form of rain or snow, or from river inflow. At higher latitudes, salinity also
is decreased by ice and snow melt.

The average salinity of the oceans is about 35, and variability in the global
distribution of surface salinity in the open ocean is shown in Figure 2.10.
Salinity valúes closely follow the curve for evaporation minus precipitation
shown in Figure 2.11. Note that the highest salinity valúes are found at about
20-30° latitude in both hemispheres, in areas having high evaporation and
low precipitation. Low salinities are found in polar areas, which have high
precipitation as well as melting ice, and in areas influenced by polar water.

Certain marine areas have salinities outside the range of those in the open
ocean. These generally occur in inshore and shallow areas that are exposed
to Coastal runoff or river inflow, or that have limited mixing with the open

Figure 2.10 The mean annual global distribution of surface salinity. The lines connecting
points of equal salinity are isohalines.

sea. The salinity ranges given below roughly characterize particular types of
marine environments:

Open ocean
Shallow Coastal areas
Estuaries

Semi-enclosed seas
(e.g. Baltic Sea)

Hypersaline environments
(e.g. Red Sea; tropical Coastal

The range of salinity in surface waters is much greater than that in deeper
layers because fluctuations result primarily from sea surface-atmosphere
interactions. Figure 2.12 displays the distribution of salinity with depth in
the Atlantic Ocean. An area where salinity changes rapidly with depth is
called a halocline. Such zones exist in low and mid-latitudes and lie from
the bottom of the mixed layer to about 1000 m. Below this depth, salinity is
34.5-35.0 at all latitudes.

Diurnal variations in salinity are usually very small, apart from intertidal
areas or shallow lagoons where evaporation and precipitation effects may be
intense. Seasonal variations in salinity are also very small, except in inshore
shallow waters. The average annual variation in surface salinity of the open
ocean is about 0.3.

32-38 (average, 35)
27-30
0-30

brackish water

> 40
lagoons)

E-P (cm)

Figure 2.11 The distribution of average surface salinity (S, black line) piotted against the
difference between average annual evaporation and precipitation (E - P, blue line) at different
latitudes.

2.3.2 BIOLOGICA!. IMPORTANCE OF SALINITY

In most marine invertebrates and primitive fish (sharks, rays), the salt
content of the blood and body fluids is about the same as in seawater of
average salinity. In bony fish (teleosts), the salt concentration of the blood is
only about 30-50% of the ambient salinity. This has several physiological
consequences. Because there is a tendency for water to move across
semipermeable membranes from a zone of Iow salt concentration to one of
high concentration (a process called osmosis), marine teleost fish tend to
lose water and thus increase their internal salt concentration. These animáis
have evolved various physiological mechanisms of osmoregulation that
counteract this problem. Most marine fish, for example, excrete very small
quantities of uriñe and secrete salts across the gills. This type of active
transport, in which the kidneys work against the normal osmotic trend,
requires an expenditure of energy. Sea turtles, seabirds, and marine
mammals also exhibit various means of maintaining osmotic balance with
their environment.

Figure 2.12(a) A cross-section of the western
Atlantic Ocean illustrating the vertical
distribution of salinity. This general pattern is
typical of all oceans, although the details will
vary from ocean to ocean.

(b) Salinity-depth profiles along lines A and B
in (a).

The problem of osmotic balance is particularly acute in those marine
invertebrates and vertebrates that inhabit estuarine areas with rapidly
changing salinities (see Section 8.5), and in those fish that migrate between
freshwater and seawater (see Section 6.6.1). Species that can tolérate a wide
range of salinity are called euryhaline, and they may display various means
of osmotic control ranging from simple impermeability (for example, closing
of mollusc shells) to complex forms of active transpon as described above.
Those animáis that can only tolérate a narrow salinity range are called
stenohaline.

QUESTION 2.5 How could the ambient salinity affect the buoyancy of pelagic
organisms?

2.4 DENSITY

The density (mass per unit volume) of seawater is governed by temperature
and salinity (and, to a lesser extern, by hydrostatic pressure). As salinity
increases, the density increases; as the temperatura increases, the density
decreases. Salinity and temperatura are physically independent variables but,
as we have seen, they are not randomly distributed in the ocean. Global
climate establishes the temperatura and salinity distribution in the surface
layers of the ocean. Distinctive combinations of these variables are thus
developed in large volumes of water, and these easily measured
temperatura-salinity characteristics can be used to define particular water
masses. Each of these bodies of water thus forms a different type of
environment, and each supports distinctive communities of organisms.

Figure 2.13 shows the major upper-layer water masses of the world that
extend in depth to about the base of the thermocline. The definitive
temperatura and salinity characteristics of these water masses are acquired at
the surface, but once water is out of contad with the atmosphere, its
physical characteristics will only change very gradually and very slowly
through mixing with adjacent waters of different characteristics. This means
that even though water masses move both horizontally and vertically in the
ocean, each can be traced over long distances by its definitive combination
of temperatura and salinity. New water masses also are eventually formed by
mixing of waters of different origins, and these too develop their own
temperatura-salinity signaturas that indicate the amount of mixing.

The upper water layers are moved horizontally by surface currents generated
by wind systems (see Section 2.6). Vertical movements of water are
controlled in part by temperatura and salinity variations that change the
density of seawater. Figure 2.14 illustrates the relationship between
temperatura, salinity, and density. Note that density itself cannot be used to
define a water mass because different combinations of temperatura and
salinity may produce the same density.

Figure 2.13 The global distribution of major upper water masses.

East N. Pacific
Transitan Waier

' Atlantic
Central
Water A

East N Pac¡k
Central Water

Wesl N. Pacific
Central Water

Pacific Equatonai Water

S. Atlantic
Ceñirá! Waier

Inconesian

Upper Water

Wesl S. Pacific
t Central Water

j Suban tárete 5

—¡¡¡¡tfCtlC Surfe^

MU!**

Atlantic Subarclic
Upper Waier^

East S Pacific
Transffion Water

QUiSTIÜN 2*6 (aj Whai is i he densiiy of sea water having a temperature oí
9 C and a salinity of 33*5? (b) What is the densiiy of seawater wiih a
temperature of 20 C and a salinity of 36,5? Refer to Figure 2.14.

Water that is less dense than underlying layers will remain at the surface
(e.g. water in equatorial latitudes). Surface water masses that increase in
density will sink to depths determined by their densities relative to the
vertical density structure of the surrounding waters. Figure 2.15 shows the
water masses lying between about 550 m and 1500 m depth; these masses
are denser than the waters above 500 m. The densest water masses occupy
depths from below 1500 m to the seafloor.

Salinity also has the important effects of lowering the temperature at which
máximum density occurs, and depressing the freezing point of seawater.
These changes are shown in Figure 2.16; note that the temperature of
máximum density and the freezing point are the same at a salinity of about
25. As the oceans are generally more saline than this (average salinity = 35),
the density of seawater continúes to increase with decreasing temperature all
the way to the freezing point (about — 1.9°C at a salinity of 35). In contrast,
the temperature of máximum density of freshwater (salinity = 0) is 4°C, and
water becomes less dense as the temperature falls to 0°C, its freezing point.
This is a profound difference between freshwater and seawater, and it has
important ramifications on oceanic circulation and on marine life.

The densest and deepest water masses originate primarily around Antárctica,
or in the vicinity of Greenland and Iceland (Figure 2.17). During winter in
high latitudes, surface waters become colder and, because seawater density
continúes to increase to the freezing point, there is a continual sinking of
water until that point is reached. As sea-ice forms, it is less saline than the
seawater so the salinity of the water is elevated, and the density further
increased. This very dense polar water sinks and flows toward the Equator
(Figure 2.17) at intermediate depths (Antarctic Intermedíate Water and North
Atlantic Deep Water) or along the seafloor (Antarctic Bottom Water).
Antarctic Bottom Water in particular penetrates far into the northem parts of

Figure 2.14 A T-S diagram showing the
relationship between temperature (T), salinity
(S) and density. For convenience, the density
contours are shown as lines of equal valúes of
(sigma-t) where a t = (density - 1) x 1000.
Therefore a density of 1.02781 g crrr 3 has a
a, = 27.81.

E. Atlantic Subarctic
/ I ni Water

Arctic Intermedíate Water

Mediterranean
/ Water

^Pacific Subarctic / . jj
I nte r med ¡ate Water (o rrnan

^_ I J-

tofermedi

liate Watnr

IredSea-Persia

intermedíate W¡

f Antarctic
Intermedíate Water

Antarctic Intermedíate Water

Antarctic intermedíate Water

East S Pacific
Intermedíate Water

Figure 2.15 The global distribution of
intermedíate water masses lyíng between about
550 and 1500 m depth. The source regions of
the water masses are indicated by dark blue.

the Atlantic and Pacific oceans. Deep water will eventually be returned to
the surface by wind-driven mixing, and thus there is a continuous, but very
slow (on the order of several hundred to a thousand years), cycling between
surface and deep waters.

In températe latitudes during winter, cooling surface water over the deep
open ocean continúes to sink and never reaches the freezing point. Thus the
ocean surface remains ice-free, except in shallow marginal seas like the Gulf
of St. Lawrence off eastem Cañada. By contrast, the water in freshwater
lakes and ponds may cool to 4°C (its temperature of máximum density) from
top to bottom. With additional cooling, the surface layer in lakes becomes
lighter and fioats, and vertical circulation ceases. Then the surface water can
continué to cool to the freezing point, with ice formation closing the surface
relatively easily.

Figure 2.16 Temperatures of freezing and

máximum density of water as a function of QUESTIÜN 2,7 Huw do voy tfnnk ihe relatively low sahnity ( < 34.5) of

salinity. Arctic surface water would affect freezing? Kefer to Figure 2.16.

When dense water sinks from the surface, water moves horizontally into the
región where sinking is occurring, and elsewhere water rises to complete the
cycle. The horizontal or vertical movement of water is referred to generally
as advection. Because water is a fixed quantity in the oceans, it cannot be
accumulated or removed at given locations without movement of water
between these regions. The sinking of water is called downwelling; upward
movements of water are called upwelling. Downwelling transports
oxygen-rich surface water to depth; upwelling retums essential nutrients (e.g.
nitrate, phosphate) to the euphotic zone where they can be utilized by plants
to produce organic materials. Because upwelling is so important for marine
productivity, it is discussed in more detail in Section 3.5.

Figure 2.17 A cross-section of the Atlantic Ocean showing the formation and movement of
intermedíate and deep-water masses from polar regions. AABW, Antarctic Bottom Water;
AAIW, Antarctic Intermedíate Water; and NADW, North Atlantic Deep Water.

2.5 PRESSURE

Figure 2.18 The relationship between pressure
and depth. Both scales are logarithmic simply
to accommodate the range of numbers.

Hydrostatic pressure is another physical environmental factor affecting life
in the sea. Pressure is determined by the weight of the overlying water
column per unit area at a particular depth. For the purposes of this book, the
relationship between pressure and depth is considered to be effectively linear
although, in fací, pressure is also influenced by density which increases with
depth.

Pressure can be measured in a variety of units. Figure 2.18 expresses
pressure in newtons per square metre. At 10 m depth, the pressure is
10 5 N m -2 ; this is roughly equivalent to 1 atm or 1 bar, which are the units
conventionally used by biologists (and scuba divers). It is easiest to
remember that, with increasing depth, pressure increases by 1 decibar every
metre (= 10 4 N m~ 2 ), or by 1 atm every 10 m.

No matter which units of pressure are used, it is apparent that many marine
organisms inhabiting deep waters are subject to very high pressures. In the
deepest ocean basins, organisms exist at pressures exceeding 1000 atm.
There are also marine animáis that daily migrate vertically over distances of
several hundred metres and thus experience considerable pressure changes.

Because of the difficulties in collecting deep-sea species under pressure and
maintaining thern at their in situ pressures in the laboratory, the biological
effects of pressure remain somewhat uncertain. It has also been difficult to
sepárate the effects of hydrostatic pressure on the metabolism of deep-sea

animáis from those effects due to living in a low temperature and dark
environment. It is known, however, that liquids can only be slightly
compressed under high pressure, but that gases are highly compressible. This
means that animáis with gas-filled structures, like the swim bladders of some
fish, may be markedly affected by pressure change, whereas animáis lacking
these structures may be more tolerant of depth change. Air-breathing marine
mammals, with gas-filled lungs, have evolved a variety of special anatomical
and physiological adaptations that permit them to make deep dives. More
generally, experiments have shown that many planktonic organisms are
sensitive to pressure and will respond to laboratory-controlled pressure
changes by swimming upward or downward. Animáis living permanently in
the deep sea do not have gas-filled organs and may have special biochemical
adaptations to living under high pressures.

Certain animáis (both benthic and pelagic species) do inhabit wide depth
ranges in the sea and these are considered to be eurybathic. Other species
are intolerant of pressure change and remain restricted to narrow depth
ranges; these are the stenobathic species. Indeed, some stenobathic forms
are restricted to deep areas and seem to require high pressures for normal
development.

2.6 SURFACE CURRENTS

The major surface currents in the ocean (Figure 2.19) are primarily
wind-driven and thus closely related to the major wind systems. However,
the eastward rotation of the Earth modifies the direction of water movement
by deflecting currents to the right in the Northern Hemisphere, producing a
tendency to clockwise circulation patterns. In the Southern Hemisphere, the
deflection is to the left, and major currents move counterclockwise.

Figure 2.19 illustrates the major clockwise gyres in both the North Atlantic
and North Pacific oceans. North of the Equator, persistent north-east trade
winds forcé water westward to form a North Equatorial Current in both
oceans. When this water reaches continental land masses, it turns northward
as the Gulf Stream in the Atlantic and as the Kuroshio in the Pacific. At
about 40°N, dominant westerly winds assist returning eastward-flowing
currents. The circuits are completed by water fiowing southward to form the
Canaries Current in the Atlantic, and the California Current in the Pacific.

The oceanic gyres flow counterclockwise in the Southern Hemisphere,
forming mirror images of their northern counterparts. South-east trade winds
generate westward-flowing South Equatorial Currents, most of which are
deflected southward (left) along eastern South America (Brazil Current) and
Australia (East Australia Current). Water flows northward along western
Africa in the Atlantic and along Chile and Perú in the Pacific before
rejoining the South Equatorial Currents.

In all of these gyres, currents are narrower, deeper, and faster along the
western edges of the oceans compared with those along the eastern margins.
For example, the Gulf Stream and Kuroshio are western boundary currents
fiowing at velocities up to 200 cm s -1 , roughly ten times faster than eastern
boundary currents (< 20 cm s" 1 for the Canaries or California currents). As
these large volumes of water circuíate, they mix with other water bodies and
their characteristics change gradually. Note, for example, the joining and

[uatorlal Counter-Current

ÍEquatorial Curfent

Brazil

Current

Aguihas

'Curre*'

Current [ V \

// ¿ FaWafids_
^ J durrgnt —

r* myi wsr 1 20*1^100 o x^,w° 60 ° 40 °w 20° / / jb° ,20^

í^m

<>\ a- Alaska / '•rvS ( .V-W (

£pp¡T

> 'S California G¡j!f %^'^'é^-JS;

'N. Pacific Curren^ vJk Streanyjjír ir

- -- -^ \ <\ Flarida ''X * >]V¿^

//ss

- ^,/ > ) Guinea

¡c^Gkgürriftolar Current (West'Wind Dri

Antarctic

Dritt>-V^

Figure2.19 The major surface currents oí the oceans in northern winter. Dashed arrows indícate
cool currents; solid arrows show warm currents.

mixture of coid, low-salinity Laborator Current water with the warm, high
salinity Gulf Stream (Figure 2.19 and Colour Píate 7).

CUESTION 2*8 Figure 2.19 shows one eastward-flowing surface current ihat is
unim peded by Land barriers and makes a complete Circuit a round the
world, connecting the Atlantic, Pacific, and ludían oceans. What is the ñame
of i his cur re n t a n d whe re i s i t 1 oca t ed?

2.6.1 BIOLOGICA!. SIGNIFICANCE OF CURRENTS

The issue of ocean circulation is the subject of physical oceanography, and
the description of generating forces for surface currents given here is far
from complete. However, the basic pattern of circulation is presented here
because water movements greatly influence biological productivity. As
currents meet and mix, or meet continental land masses or major rivers, or
move over shallower depths, various types of vertical circulation patterns are
generated that affect the distribution of nutrients available to
phytoplankton. Section 3.5 explores the mechanisms that produce these
geographical differences in production. The patterns of ocean currents also
influence the geographical distributions of both pelagic and benthic marine
species.

The oceans form a dynamic, fluid environment moving over the surface of
the Earth, and this creates one of the most difficult problems for biological
oceanographers because it is impossible to follow the same population or

community of pelagic organisms for any appreciable time period. A body of
water sampled at a specific locality will not be the same body of water 1
hour later at the same position. Even if a patch of surface water is marked
by a floating buoy or drogue, the resident animáis in the underlying water
column will change as they vary their depth positions. By doing so, they
enter water moving at different speeds and in different directions relative to
the surface. This is why many biological processes that require sampling over
longer time spans, such as growth rates of zooplankton, are measured in the
laboratory using captive animáis, or are inferred using indirect techniques.

2.7 SUMMARY OF CHAPTER 2

1 The amount of sunlight arriving at the sea surface varies with time of
day, season, and weather. Approximately 50% of the solar radiation
penetrating the sea surface is within the visible spectrum (about
400-700 nm), and these are approximately the same wavelengths used in
plant photosynthesis. The intensity of photosynthetically active radiation
(PAR) at the sea surface ranges from zero (in darkness) to about

2000 fiE irr 2 s ' 1 with the Sun directly overhead.

2 Different wavelengths of light are absorbed and scattered at different
depths in water, and they have different extinction coefficients, with red light
being attenuated most rapidly and blue light penetrating deepest in clear
water. The depth to which any wavelength penetrates depends partly on the
amount of suspended particles and chlorophyll in the water.

3 Three ecological zones have been defined, based on the penetration of
light in seawater. The euphotic zone is that región where light is sufficient
for the growth of plants, and it extends from the surface to a máximum of
about 150 m in the clearest oceanic water. The lower boundary is defined by
the compensation light depth, where only enough light is present for
photosynthesis to balance plant respiration over 24 hours. The disphotic zone
is dimly lighted; there is sufficient light for visión, but too little for plant
production. The deepest and largest zone is the aphotic zone, a región of
darkness extending to the seafloor where the only light emanates from the
bioluminescence of certain animáis.

4 Infrared wavelengths are absorbed within the first few metres of the sea
surface and are the primary heat source of the oceans. Sea surface
temperatures vary with latitude and fluctuate seasonally but remain within a
modérate range of about 40°C to — 1.9°C, the freezing point of water with a
salinity of 35.

5 In many parts of the ocean, there is thermal stratification consisting of an
upper mixed layer of water of almost homogeneous temperature; a región of
rapid temperature decrease known as the permanent thermocline; and an
underlying coid deep layer of water formed originally at the surface in polar
regions.

6 In mid-latitudes where seasons are pronounced, seasonal thermoclines are
formed in the surface layer during spring and summer. These zones of steep
temperature change are established because increased solar radiation elevates
surface temperatures at a time when lessened winds reduce the amount of
mixing in the water.

7 The average salinity of the open ocean is about 35 parts per thousand by
weight, with ten major ions making up about 99.99% of all the dissolved
substances in the oceans. In inshore or isolated areas with little water
exchange, salinity may vary from about 5 to 25 in brackish waters, to more
than 40 in such hypersaline areas as the Red Sea and some shallow lagoons.
Variations in salinity are primarily caused by evaporation (which elevates
salinity) and precipitation (which decreases salinity).

8 Whereas total salinity is variable, the major dissolved ions are not
significantly affected by biological or Chemical reactions and the relative
proportions of these dissolved constituents remain constant.

9 The combined properties of salinity and temperature are used to define
water masses. Each of these large bodies of water has a discrete origin and
forms a distinctive environment, supporting a distinctive community of
pelagic organisms.

10 Salinity, temperature, and pressure establish the density of seawater.
Changes at the sea surface that result in higher density will lead to
downwelling of that water. Very dense water formed at high latitudes sinks
to form the bottom water masses of the oceans, and this process is important
in maintaining oxygen levels at all depths. Upwelling of water is partly
caused by wind-driven mixing and is of importance in returning biologically
essential elements to surface waters, where they are used by plants in
photosynthesis.

11 The salt content of the sea lowers the temperature of máximum density
and depresses the freezing point of seawater relative to freshwater. This not
only results in the winter downwelling of polar water (see 10 above), but it
also prevents sea-ice formation except in polar areas and in shallow
high-latitude marginal seas.

12 Oceanic surface currents are generated by global wind systems, and
their direction is modified by the Earth's rotation. This results in large
clockwise-moving gyres in the northern oceans and anticlockwise gyres in
the Southern Hemisphere. The patterns of movement and mixing of these
currents produces geographic regions of differing biological productivity.
Horizontal transport of water also establishes the geographic distribution of
many marine species.

13 Hydrostatic pressure effectively increases linearly with depth, at a rate
of 0.1 atm m -1 . In the deepest areas, organisms live at pressures exceeding
1000 atm.

Now try the following questions to consolídate your understanding of this
Chapter.

CUESTION 2J In Seelion 24,2, compensador) light intensities de) for
phytoplankton are given as ranging between I and 10 /¿E What are

ihese valúes in watts m : ? tRefer to Appendix 2 for conversión factors.)

CUESTION 2,10 The majority of marine animáis (boih pelagic and benthic) are
poikilothertnic, whereas many land animáis (birds, mammais) are
homoiothermic species. Can yoti think of a reason to explaín this difference?

QOESTIOH 2.11 Refer lo ihe global ranges of suríace saliniiy shovvn ín
Figure 2J 1. (a) Explain ihc low salinily valué (34.5) ui ihe Equator.

(b) Why is salinily higher in surface waters of thc Antarctic (ca. 57 $
lalitude) comparecí to ihe Arctic (57 N)7

QUESTION 2.12 Refer lo Figure 2.14, Whíeh combinaron of high or low
le ni pe ral u re and high or low saliolty would produce water of grealesl
densiiy?

QUESTION 2.13 Review whal you have learned about abiolie env ironmemal
factors ín ibis Chapier and describe the deep-sea environmeni be low 2000 m
in le mis of light, salinily. tempera ture, pressure. and reí al i ve densiiy.

CHAPTER 3

PHYTOPLANKTON AND PRIMARY
PRODUCTION

The great majority of the plants in the ocean are various types of planktonic,
unicellular algae, collectively called phytoplankton. Although some
phytoplankton are large enough to be collected in fine-mesh nets, many of
these microscopic plants can only be collected by filtering or centrifuging
sizable volumes of seawater. There are also macroscopic floating algae in
some oceanic areas, Sargassum in the Sargasso Sea being a well known
example, but they are relatively restricted in locality. Similarly, the benthic
species of algae, including attached macroscopic seaweeds, are limited in
distribution to coastal, shallow areas because of the rapid attenuation of light
with depth. In contrast, phytoplankton are present throughout the lighted
regions of all seas, including under ice in polar areas. Because the
phytoplankton are the dominant plants in the ocean, their role in the marine
food chain is of paramount importance.

Table 3.1 A taxonomic survey of the marine phytoplankton.

Class

Common

ñame

Area(s) of
predominance

Common

genera

Cyanophyceae

Blue-green

Tropical

Oscillatoria

(Cyanobacteria)

algae (or

blue-green

bacteria)

Synechococcus

Rhodophyceae

Red algae

Coid températe

Rhodella

Cryptophyceae

Cryptomonads

Coastal

Cryptomonas

Chrysophyceae

Chrysomonads

Coastal

Aureococcus

Silicoflagellates

Coid waters

Dictyocha

Bacillariophyceae

Diatoms

All waters.

Coscinodiscus

(Diatomophyceae)

esp. coastal

Chaetoceros

Rhizosolenia

Raphidophyceae

Chloromonads

Brackish

Heterosigma

Xanthophyceae

Yellow-green

algae

Very rare

Eustigmatophyceae

—

Estuarine

Very rare

Pry mnesiophyceae

Coccolithophorids

Oceanic

Emiliania

Prymnesiomonads

Coastal

¡sochrysis

Prymnesium

Euglenophyceae

Euglenoids

Coastal

Eutreptiella

Prasinophyceae

Prasinomonads

All waters-

Tetrasalmis

Micromonas

Chlorophyceae

Green algae

Coastal

Rare

Pyrrophyceae

Dinoflagellates

All waters.

Ceratium

(Dinophyceae)

esp. warm

Gonyaulax

Pro tope r id i n i u ni

This table is included for completeness and information, but it is not necessary to
remember all details. It is important to note the diversity shown among the
phytoplankton.

3.1 SYSTEMATIC TREATMENT

Approximately 4000 species of marine phytoplankton have been described,
and new species are continually being added to this total. A taxonomic list
of the major types of phytoplankton is given in Table 3.1, but only the better
known groups are considered in some detail below.

3.1.1 DIATOMS

Diatoms (Figure 3.1; Colour Plates 1 and 2) belong to a class of algae
called the Bacillariophyceae. They are among the best studied of the
planktonic algae and are often the dominant phytoplankton in températe and
high latitudes. Diatoms are unicellular, with cell size ranging from about
2 ¡im to over 1000 /xm, and some species form larger chains or other forms
of aggregates in which individual cells are heid together by mucilaginous
threads or spines. All species have an external skeleton, or frustule, made of
silica and fundamentally composed of two valves. Silica in the skeleton

Figure 3.1 Diatoms. (a) a typical Chain of
Chaetoceros faciniosus, (b) C. iacirtiosus Chain
with resting spores; (c) Nitzschia pungens Chain
of dividing cells; (d) Thalassiosira grávida Chain;
(e) Coscinodiscus showíng the two valves of the
frustule; (f) Coscinodiscus wailesii, lateral view;
(g) Chaetoceros sociaiis chains in a gelatinous
colony formation; (h) a chain of Asterioneila
japónica, and (i) Skeletonema costatum. (scales
in mm)

parent cell

/ \

asexual división

J— \

0 43

offspring of
differing size

“i

diminution of size

critical minimal size

sexual reproduction
and the

formation of auxospores
Figure 3.2 The life cycle of diatoms.

makes up 4-50% of the dry weight of the cell. The frustule is usually
sculptured into patterns of spines, pores, channels, and/or ribs which are
distinctive to individual species. Diatoms have been abundant in the seas
since the Cretaceous (about 100 million years ago) and, over geological
time, sedimented frustules have formed seafloor deposits called
diatomaceous ooze.

Two types of diatoms are recognized: the pennate and centric forms. Pennate
diatoms have elongate shapes and are mostly benthic, but the few planktonic
genera such as Nitzschia (Figure 3.1c) may be abundant in some regions.
Centric diatoms have val ves that are arranged radial ly or concentrical ly
around a point, and they are much more common in the píankton, with
somewhat over 1000 species. Chaetoceros , Coscinodiscus, Skeletonema , and
Thalassiosira are all common centric genera, some of which are illustrated
in Figure 3.1.

Planktonic diatoms do not have any locomotor structures and are usually
incapable of independent movement. Because it is essential for diatoms and
other phytoplankton to remain in lighted surface waters in order to carry out
photosynthesis, these algae exhibit a variety of mechanisms which retard
sinking. These inelude their small size and general morphology, as the ratio
of cell surface area to volume determines frictional drag in the water.

Colony or chain formation also increases surface area and slows sinking.
Most species carry out ionic regulation, in which the internal concentration
of ions is reduced relative to their concentration in seawater. Diatoms also
produce and store oil, and this metabolic by-product further reduces cell
density. In experimental conditions, living cells tend to sink at rates ranging
from 0 to 30 m day -1 , but dead cells may sink more than twice as fast. In
nature, turbulence of surface waters is also important in maintaining
phytoplankton near the surface where they receive abundant sunlight.

The usual method of reproduction in diatoms is by a simple asexual división
in which the cell forms two nuclei, the two halves of the frustule sepárate,
and each resulting daughter cell grows a new inner valve of the frustule
(Figure 3.2). This can result in the two new cells being of slightly unequal
size, the one receiving the inner half of the original frustule being slightly
smaller than the cell formed from the outer valve. Asexual división can lead
to very rapid population growth under optimal conditions. However, with
repeated divisions, there may be a diminution in size of some of the progeny.

When a diatom reaches a certain critical minimal size, it undergoes sexual
reproduction by forming a cell that lacks a siliceous skeleton and contains
only half of the genetic material. Such cells fuse to form a zygote, and this
swells to produce an auxospore. Subsequently, a larger cell is formed that
ultimately produces a frustule of the normal shape and size. Sexual
reproduction in diatoms does not necessarily require a reduction in size of
the cell, however.

Some diatoms, particularly neritic species living in relatively shallow water,
produce resting spores (Figure 3.1b) under adverse environmental
conditions. These form when the protoplasm of a normal cell becomes
concentrated and surrounded by a hard shell. This heavy spore sinks to the
bottom and remains dormant until favourable conditions are restored, in
which case it is capable of becoming a normal planktonic cell.

3.1.2 DINOFLAGELLATES

The second most abundant phytoplankton group following the diatoms is
composed of algae belonging to the Pyrrophyceae, and commonly referred to
as dinoflagellates (Figure 3.3; Colour Píate 3). Most of these unicellular
algae exist singly; only a few species form chains. Unlike the diatoms,
dinoflagellates possess two flagella, or whiplike appendages, and are
therefore motile.

Different species of dinoflagellates utilize different energy sources. Only
some dinoflagellates are strictly autotrophic, building organic materials and
obtaining all their energy from photosynthesis. Other species carry out
heterotrophic production; that is, they meet their energy needs by feeding
on phytoplankton and small zooplankton. Indeed, about 50% of the
dinoflagellates are strict heterotrophs that lack chloroplasts and are incapable

Figure 3.3 Dinoflagellates. (a) Two views of Prorocentrum marinum, (b) Prorocentrum
micans, (c) P. micans dividing; (d) Protoperidinium crassipes, (e) Gymnodinium abbreviatu/rr,
(f) Dinophysis acuta, and (g) Gonyaulax fragilis. (All scale bars represent 0.02 mm.)

DESMOPHYCEAE

of carrying out photosynthesis; these species form part of the zooplankton
and are considered in Section 4.2. Some dinoflagellates are mixotrophic,
and are capable of both autotrophic and heterotrophic production (see also
Section 4.2). Still other dinoflagellate species are parasitic or symbiotic (e.g.
see Section 8.6.4). There are an estimated 1500 to 1800 species of
free-living, planktonic dinoflagellates.

Conventionally, dinoflagellates are divided into thecate species, which have a
relatively thick cellulose cell wall called a theca, and naked forms which
lack this structure. Taxonomically, the dinoflagellates are separated into the
Desmophyceae and the Dinophyceae. The former is a small group in which
the species are characterized by having both flagella arising from the anterior
end of the cell (Figure 3.3a, b). The cell wall is composed of two
longitudinal valves that sepárate during asexual división to form two new
cells of equal size (Figure 3.3c). Prorocentrum is a common planktonic
genus belonging to the Desmophyceae.

The majority of planktonic dinoflagellate species form the Dinophyceae
(Figure 3.3d-g; Colour Píate 3), and the majority of these are thecate. In all
of them, the cell is divided into an anterior and posterior half by a transverse
groove known as a girdle. The flagella are so arranged that one extends
posteriorly from the cell, and the other wraps transversely around the cell in
the girdle región. In those species with a theca, the cell wall is divided into a
number of sepárate cellulose plates that are ornamented with pores and/or
small spines. Common thecate genera inelude Ceratium , Protoperidinium ,
Gonyaulax , and Dinophysis. Gymnodinium is a common naked form
belonging to the Dinophyceae.

Reproduction in dinoflagellates is normally by simple asexual división, with
the cell dividing obliquely to form two cells of equal size. The theca may
divide, with each new cell forming a new half, or the theca may be lost
before división, in which case each new cell forms an entirely new cell wall.
Asexual división can lead to rapid population development when conditions
favour these algae. Dinoflagellates often become abundant in summer or
autumn, following blooms of diatoms, as they are better adapted at living
under lower light conditions and in nutrient-impoverished water. This is
partly because dinoflagellates are capable of moving vertically in the water
column; during the day they can carry out photosynthesis in sunlit surface
waters that have been stripped of nutrients by fast-growing algae, and at
night they may move deeper to take advantage of higher nutrient
concentrations. For the same reason, dinoflagellates are usually the most
numerous of the phytoplankton in stratified, nutrient-poor tropical and
subtropical waters (see Section 2.2.2 and Figure 3.9).

Sexual reproduction also occurs in at least some species of dinoflagellates.
This may lead to the formation of thick-walled, dormant cysts that settle on
the seafloor, where they can survive for years. When triggered by
environmental change, the cysts germinate to produce swimming cells.

Phytoplankton blooms develop when a species suddenly increases greatly
in numbers under favourable conditions. In some circumstances, the rapid
reproduction of dinoflagellates results in such high densities of organisms
that their reddish-brown pigment visibly colours the water, producing
so-called red tides (Colour Píate 4) (see also Section 3.1.3, Cyanophyceae).
This red water may be caused by very high concentrations of innocuous
species of dinoflagellates, or of species that contain potent toxins (see

below). In any case, red tides begin with a sudden increase in numbers of
the dinoflagellate. The water becomes noticeably coloured when
concentrations reach about 200000 to 500000 cells 1 _I and, as the bloom
develops, concentrations may exceed 10 8 cells l~ l . When essential nutrients
are exhausted by the dinoflagellates and the bloom decays, the bacterial
decomposition of large amounts of organic material depletes the available
oxygen and fish may die as a result of the lowered oxygen concentrations.
The development of anoxic conditions is not exclusively a property of
dinoflagellate blooms; such conditions can also occur following large blooms
of other types of phytoplankton.

Some red tides are caused by certain species of Alexandrium , Pyrodinium ,
and Gymnodinium which produce a variety of neurotoxins collectively
referred to as saxitoxin, which is 50 times more lethal than strychnine and
10000 times more deadly than cyanide. Even when present in concentrations
too low to colour the water, these dinoflagellates can be poisonous to certain
animáis and to humans. While the dinoflagellates are growing and
reproducing, they build up saxitoxin in their cells and some of this is
released into the water. The toxic dinoflagellates are also ingested by some
zooplankton and by filter-feeding shellfish like clams, mussels, scallops, and
oysters. Zooplankton and shellfish accumulate and concéntrate saxitoxin in
their own tissues, where it may be retained for considerable periods without
harmful effects. However, vertebrates, such as fish, are sensitive to saxitoxin
and may die from eating contaminated zooplankton. In serious outbreaks,
seabirds and even dolphins and whales may also perish by accumulating
saxitoxin from their food.

The minimum lethal dose of saxitoxin for humans is 7 to 16 ¡xg kg -1 of
body weight, and eating a single contaminated clam may be enough to cause
death from paralytic shellfish poisoning (or PSP). Saxitoxin is heat stable,
so cooking of the shellfish does not destroy the potency of this neurotoxin.

In North America within historie medical times, about 1000 cases of shellfish
poisoning have been recorded, with about one-quarter of these resulting in
death. One of the earliest recorded cases occurred off the west coast of
Cañada on 15 June 1793, when one death and four illnesses resulted from
crew on Captain George Vancouver’s ship eating toxic mussels. In 1799, 100
men on a Russian expedition off Alaska died from eating mussels. Paralytic
shellfish poisoning remains a problem on both coasts of North America, in
Central America, and the Philippines; it also occurs in Europe, Australia,
South Africa, and Japan. In 1987, three human fatalities and 105 cases of
acute poisoning were reported in eastern Cañada as the result of consuming
toxic mussels. In this case, the neurotoxic compound was identified as
domoic acid that originated in a diatom, Pseudonitzschia , which had
previously been considered harmless. Developed countries typically have
monitoring programmes that permit the closure of contaminated shellfish
beds (natural or cultivated) when toxins are detected in the water or in
shellfish tissues; the incidence of sickness and fatalities from algal-derived
shellfish poisoning is higher in Coastal developing countries.

A related health problem, ciguatera fish poisoning (or CFP) is found in
tropical and subtropical countries, where certain species of toxic
dinoflagellates live attached to seaweeds. Fish that feed on seaweeds also
ingest the dinoflagellates and accumulate toxin in their tissues, and this is
passed on through the food web to carnivorous fish, and eventually to
humans who consume contaminated fish. Symptoms range from headache

and nausea in mild cases to convulsions, paralysis, and even death in severe
cases. It is estimated that CFP causes more human illness than any other
kind of toxicity originating in seafood, with 10000 to 50000 individuáis
being affected each year.

3.1.3 OTHER PHYT0PLANKT0N

Coccolithophorids (Colour Píate 5) are unicellular phytoplankton that form
part of the nanoplankton (refer to Figure 1.2), with most of the 150 or so
species being smaller than 20 ¡jl m. Their outstanding characteristic is an
external shell composed of a large number of calcareous plates called
coccoliths. The shape and arrangement of the coccoliths can be used to
identify species. The coccoliths accumulate in bottom sediments, and they
are the major constituent of the uplifted sediments known as chalk, which
forms the famous White Cliffs of Dover. Like the dinoflagellates,
coccolithophorids possess two flagella, although they may have a life cycle
which ineludes an alternation with a non-motile stage lacking flagella.
Although coccolithophorids can be found in neritic as well as in oceanic
waters (see Figure 1.1), and at times are near the surface, the majority of
species occur in warmer seas and thrive in reduced light intensities; some
species reach máximum abundance at depths of about 100 m in clear,
tropical, oceanic water. However, Emiliania huxleyi is probably the most
widespread coccolithophorid in the sea, and it is present in all oceans except
the polar seas. Emiliania sometimes forms enormous blooms, one having
been measured to cover approximately 1000 km by 500 km of sea surface in
the North Atlantic Ocean — or an area roughly the size of Great Britain.
Reproduction in coccolithophorids is by longitudinal división, with the shell
being divided and afterwards reformed into a whole by each new cell.
However, life histories in this group are complex and may involve several
different types of stages.

Allied with coccolithophorids in the algal group Prymnesiophyceae are
several other important phytoplankton which lack coccoliths and are
superficially very different in appearance. These inelude such unicellular and
motile genera as Isochrysis , a small alga commonly cultured in the
laboratory, and Phaeocystis , which forms large gelatinous colonies that can
foul fish nets and also beaches when washed ashore. Prymnesium is
characteristic of low salinity water and can be a major cause of mortality in
farmed salmón along the Norwegian coast because it interferes with gas
exchange across the gills of the fish.

The best known marine forms of the Chrysophyceae, or golden-brown algae,
are the silicoflagellates with an internal skeleton formed of siliceous spicules
(Colour Píate 6). These uniflagellate organisms are small (10-250 /xm) and
contain very numerous yellow-brown chloroplasts. Only a few species of
silicoflagellates are known, and these are usually most abundant in colder
waters.

Numerous species of small, naked, flagellated phytoplankton also make up
other taxonomic divisions (Table 3.1). Some of these species are truly rare,
but many remain poorly known because of the difficulties in collecting and
preserving very small cells (including picoplankton of 0.2-2 /xm) which do
not have rigid skeletal structures. Some flagellates that survive collection
will disintegrate during filtration or when placed in preservatives.
Nevertheless, some of these minute phytoplankton can be very abundant and
important in ecological eyeles.

Some of the smallest, and also some of the largest, species of phytoplankton
belong to the Cyanophyceae or Cyanobacteria (also known as blue-green
algae, or blue-green bacteria). A single genus, Oscillatoria (formerly called
Trichodesmium) is well known, and -is important in the tropical open ocean.
At times this alga exists in single long filaments formed by chains of cells;
at other times, the filaments clump together to form macroscopic bundles of
several millimetres in diameter. Interest has been directed toward this genus
as the species are capable of utilizing and fixing dissolved gaseous nitrogen
(N 2 ), unlike other phytoplankton which can only utilize combined forms of
nitrogen such as nitrate, nitrite, and ammonia. The attribute of nitrogen
fixation may explain the relative success of Oscillatoria in tropical waters
which typically have low concentrations of the nitrogen sources normally
utilized by other algae. Nitrogen fixation does not seem to be a physiological
feature of Synechococcus , another genus of marine cyanobacteria.
Synechococcus is of picoplankton size (refer to Figure 1.2); it occurs
abundantly in the euphotic zone of both Coastal and oceanic waters of
températe and tropical oceans. Concentrations of Synechococcus may reach
up to 10 6 cells mi -1 and, at such high concentrations and in the absence of
larger phytoplankton, this single genus can play a major role in the primary
productivity of the sea. Recently scientists have discovered even smaller
(0.6-0.8 ¡x m diameter) photosynthetic organisms called prochlorophytes,
which are closely related to the cyanobacteria and occur in both Coastal and
oceanic waters. Although few ecological studies have been made of these
organisms, the genus Prochlorococcus apparently contributes to a significant
fraction of the total primary production in the oceanic equatorial Pacific.

OH ESTIO N 3,1 Assuming a spherical shape, how many Synechococcus cells of
1 ftm diameter are equívalent ín volume to a single dinofiagellate ce 11 of 50
gm diameter?

3.2 PHOTOSYNTHESIS AND PRIMARY PRODUCTION

Phytoplankton are the dominant primary producers of the pelagic realm
converting inorganic materials (e.g. nitrate, phosphate) into new organic
compounds (e.g. lipids, proteins) by the process of photosynthesis and
thereby starting the marine food chain. The amount of plant tissue build up
by photosynthesis over time is generally referred to as primary production,
so called because photosynthetic production is the basis of most of marine
production. As we will see later in Sections 5.5 and 8.9, there are other
types of primary production that are carried out by bacteria capable of
building organic materials through chemosynthetic mechanisms, but these
are of minor importance in the oceans as a whole.

Although a number of steps are involved, the Chemical reactions for
photosynthesis can be very generally summarized as:

photosynthesis

(requiring sunlight)

6C0 2 + 6H 2 0 - C 6 H l2 0 6 + 60 2

carbón dioxide water carbohydrate oxygen

respiration

(requiring metabolic energy)

Carbón dioxide utilized by the algae can be free dissolved CO 2 , or CO 2
bound as bicarbonate or carbonate ions (see also Section 5.5.2). The total
carbón dioxide (all three forms) is about 90 mg CO 2 1 _1 in oceanic waters,
and this concentration is sufficiently high so that it does not limit the amount
of photosynthesis by phytoplankton. This type of production, involving a
reduction of carbón dioxide to produce high-energy organic substances, is
also called autotrophic production; autotrophic organisms do not require
organic materials as an energy source. Note that this process not only results
in the production of plant carbohydrate, but it also produces free oxygen
(which is derived from the water molecule, not from the carbón dioxide).
The reverse process is respiration, in which there is an oxidative reaction
that breaks the high-energy bonds of the carbohydrates and thus releases
energy needed for metabolism. All organisms, including plants, carry out
respiration. Whereas photosynthesis can proceed only during periods of
daylight, respiration is carried out during both light and dark periods.

wavelength (nm)

Figure 3.4(a) The absorption spectrum of
chlorophyll a .

(b) The absorption spectra of the accessory
pigments fucoxanthin (a xanthophyll) and
phycocyanin and phycoerythrin (phycobilins).

Solar energy is used to drive the process of photosynthesis, and the
conversión of radiant energy to Chemical energy depends upon special
photosynthetic pigments that are usually contained in chloroplasts of the
algae. The dominant pigment is chlorophyll a , but chlorophylls b , c, and d
plus accessory pigments (carotenes, xanthophylls, and phycobilins) are also
present in many species and some of these pigments can also be involved in
this conversión. All of these photosynthetically active pigments absorb light
of wavelengths within the range of about 400-700 nm (PAR), but each
shows a different absorption spectrum. Figure 3.4a gives the absorption
spectrum of chlorophyll a, the most commonly occurring pigment; máximum
absorption takes place in the red (650-700 nm) and blue-violet (450 nm)
range. Figure 3.4b shows the absorption spectra of several accessory
pigments. It is often these accessory pigments that dominate over the green
colour of chlorophyll, and therefore many phytoplankton appear to be
brown, golden, or even red in colour.

QUESTiQN 3.2 Some planktonic (and benthic) algae contain large amounts of
accessory pigments as well as chlorophyll. Refer to Figures 3.4a, b and
Figure 2.4 and suggest how these pigments may be ecologically importan!
for the algae concerned.

When chlorophyll or other photosynthetically active pigments absorb light,
the electrons in the pigments molecule acquire a higher energy level. This
energy in the electrons is then transferred in a series of reactions in which
ADP (adenosine diphosphate) is changed to higher energy ATP (adenosine
triphosphate), and a compound called nicotinamide adenine dinucleotide
phosphate (or NADPH 2 ) is formed. These reactions, which are entirely
dependent on light energy and involve the conversión of radiant energy to
Chemical energy, are called the light reactions of photosynthesis.

The light reactions are inextricably linked with a series of reactions that do
not require light and which are referred to as the dark reactions of
photosynthesis. They involve the reduction of CO 2 by NADPH 2 and require
the Chemical energy of ATP to produce the end producís of high-energy
carbohydrates (usually polysaccharides) and other organic compounds such
as lipids. Additionally, the reduction of nitrate (N0 3 ") yields amino acids
and proteins.

Note that in the reactions of photosynthesis, compounds are formed that
contain nitrogen and phosphorus as well as the elements supplied by carbón

dioxide and water. As with all plants, phytoplankton have absolute mínimum
requirements for these elements. Nitrogen is usually taken up by the
phytoplankton cell in the form of dissolved nitrate, nitrite or ammonia;
phosphorus is normally taken up in dissolved inorganic form (orthophosphate
ions), or sometimes as dissolved organic phosphorus. Other elements may be
required as well. Dissolved Silicon, for example, is an absolute requirement
for diatoms in producing the frustule. In addition, vitamins and certain trace
elements may also be required, with types and amounts depending upon the
species of phytoplankton. When photosynthetic species require vitamins or
other organic growth factors, the production is termed auxotrophic. In
seawater, all of the compounds referred to here are present in relatively low
concentrations that vary according to the rates of photosynthesis and
respiration and other biological activities, such as excretion by animáis or
bacterial decomposition. Therefore the concentrations of these essential
elements or substances may at times become so low as to limit the amount
of primary production. These considerations are discussed in Section 3.4.

3.2.1 METHODS OF MEASURING BIOMASS AND PRIMARY PRODUCTIVITY

Standing stock refers to the number of organisms per unit area or per unit
volume of water at the moment of sampling. For phytoplankton, this can be
measured by microscopic cell counts of preserved phytoplankton filtered
from seawater samples, and the standing stock is given in number of cells
per volume of water. However, because phytoplankton vary greatly in size,
total numbers are not as ecologically meaningful as estimates of their
biomass. Biomass is defined as the total weight (total numbers x average
weight) of all organisms in a given area or volume. It is possible to count
numbers and measure volumes of phytoplankton electronically, and this
method attempts to provide an estímate of phytoplankton biomass, although
cell volume may not always accurately reflect cell weight. Biomass is then
expressed as the total volume (total numbers x volumes — mm 3 ) of
phytoplankton cells per unit volume of water. The distinction between
standing stock and biomass is not always made evident, however, and often
the terms are used synonymously.

Another laboratory method that attempts to estímate phytoplankton biomass
determines the quantity of chlorophyll a in seawater. This method is often
used because chlorophyll a is universally present in all species of
phytoplankton, can be easily measured, and its relative abundance enables
estimates to be made of the productive capacity of the phytoplankton
community. A known volume of water is filtered, and plant pigments are
extracted in acetone from the organisms retained on the filter. The
concentration of chlorophyll a is then estimated by placing the sample in a
fluorometer to measure fluorescence, or in a spectrophotometer which
measures the extinction of different wavelengths in a beam of light shining
through the sample. The biomass is expressed as the amount of chlorophyll
a per volume of water, or as the amount contained in the water column
under a square metre of water surface.

However, the rate at which plant material is produced, or the primary
productivity, is of more ecological interest than instantaneous measures of
standing stock or biomass. The most popular method of measuring
productivity in the sea is the 14 C method. In this method, a small measured
amount of radioactive bicarbonate (HCO 3 - ) is added to two bottles of
seawater containing phytoplankton. One bottle is exposed to light and

permits photosynthesis and respiration; the other is shielded from all light so
that only respiration takes place. The amount of radioactive carbón taken up
per unit time is later measured on the phytoplankton when they are filtered
out of the original samples. This radioactivity is measured using a
scintillation counter, and primary productivity (in mg C m -3 h -1 ) is
calculated from:

rate of production

(Rl — Rd) x W
R x t

(3-1)

where R is the total radioactivity added to a sample, t is the number of hours
of incubation, R L is the radioactive count in the ‘light’ bottle sample, and R D
is the count of the ‘dark’ sample. W is the total weight of all forms of
carbón dioxide in the sample (in mg C rrT 3 ), and this is determined
independently, either by titration or from assuming a specific carbón dioxide
contení related to the salinity of the sample. The productivity is expressed as
the amount (in mg) of carbón fixed in new organic material per volume of
water (m -3 ) per unit time (h -1 ); it varies between zero and as much as about
80 mg C m -3 h -1 . This method is applied to water samples taken from a
series of depths. In order to calcúlate production throughout the euphotic
zone and to facilitate comparisons, the results obtained at different depths
may be integrated to give production in terms of the amount of carbón fixed
in the water column under a square metre of surface per day
(g C m -2 day -1 ). If the amount of carbón fixed per unit time is coupled with
chlorophyll a measurements of biomass, one obtains a measure of growth
rate in units of time (mg C per mg chlorophyll a per hour); this measure of
productivity is sometimes called the assimilation índex (see Table 3.2).

The carbón-14 method described above can be made very precise by careful
experimental techniques but, at the same time, there is reason to question its
accuracy. For example, the uptake in the dark bottle (Rd) is assumed to
represent a blank with which to correct the uptake in the light bottle (Rl).
This assumes that, except for photosynthesis, the same biological activities
go on in both the light and dark bottles; but this may not be quite true. Also,
any soluble organic material that is lost by the phytoplankton (a process
known as exudation) during the period of photosynthesis will not be
measured as it is not retained during filtration. Therefore, although the ,4 C
method is the most practical measurement of photosynthesis in the sea, it
may sometimes lead to errors.

Other techniques have been developed for measuring chlorophyll
concentration and thus relative phytoplankton abundance over large expanses
of sea. A fluorometer that produces a certain wavelength of ultraviolet light
will cause chlorophyll to emit a red fiuorescence, and this device can then
estimate the amount of chlorophyll in a volume of water. The method is very
sensitive, and a fluorometer towed from a research vessel (see Figure 4.2)
can rapidly record changes in chlorophyll concentration over large distances
of sea surface. Remóte sensing by aircraft or satellites provides even broader
spatial coverage of phytoplankton abundance. This technique is based on the
fact that the radiance reflected from the sea surface in the visible (or PAR)
spectrum (400-700 nm) is related to the concentration of chlorophyll.
Because chlorophyll is green, and water colour changes from blue to green
as chlorophyll concentration increases, the relative colour diffcrences can be
used as a measure of chlorophyll concentration (see Colour Píate 8). Satellite
measurements are not as sensitive as others and have restrictions of limited

depth penetration, but they provide useful pattems of relative plant
production on a global scale.

3.3 RADiATION AND PHOTOSYNTHESIS

The amount of light (or solar radiation) strongly affects both the amount and
rate of photosynthesis. Thus the photosynthesis occurring in a water sample
is proportional to the light intensity, as shown in Figure 3.5 where
photosynthesis increases with increasing light intensity up to some maximal
valué (P max ). At still higher light intensities, there may be a significant
decrease in photosynthesis (called photoinhibition) that is caused by a
number of physiological reactions such as shrinkage of chloroplasts in bright
light.

The point on the curve in Figure 3.5 at which the amount of respiration
exactly balances the amount of photosynthesis is called the compensation
point, and this occurs at a compensation light intensity (/ c ) which was
defined earlier (Section 2.1.2) as marking the lower boundary of the euphotic
zone. The term gross primary productivity (P g ) is used to describe the
total photosynthesis, and net primary productivity ( P n ) denotes gross
photosynthesis minus plant respiration.

The curve in Figure 3.5 can be described by mathematical equations that
closely approximate two independent series of reactions, one series (shown
by the initial slope AP/AI) being the light-dependent reactions of
photosynthesis and the other (P max ) being the dark reactions, both of which
were defined in Section 3.2. The simplest equations which describe the curve
up to P max (i.e. with no photoinhibition) are:

/W/j

Ki + [/I

(3.2)

Figure 3.5 The response of photosynthesis (P)
to changes in light intensity (I). l c ,
compensation light intensity; K¡, the
half-saturation constant, or the light intensity
when photosynthesis equals 1/2 of maximal
photosynthesis (P max ); P g , gross photosynthesis;
and P n , net photosynthesis. Absolute units not
shown because all units are species specific.

(cal cnr 2 min 11 )

and

PmaxU ~ I C ]
*/ + [/- Ic\

(3.3)

where P¿ and P n are gross and net productivity, respectively, as defined
above; and K¡ ís the half-saturation constant, or the light intensity when
P = P max /2. Af/ valúes range from about 10 to 50 jx E mr 2 s' 1 . [I] is the
amount of ambient PAR light, and f 1 — le] is the ambient PAR light less the
compensation light intensity, /c-

In the above equations, it is implied that there is a defined light response for
all algae growing under constant physiological conditions and that this
response can be described by two constants, P max and K¡. In fací, different
species have different valúes of P max and K¡ and, even within the same
species, the photosynthetic response of a cell to light can change over time
(e.g. over the course of a day from bright light near the surface to shade
adaptation deeper in the water column). In general, the initial slope of the
curve (AP/A/) in Figure 3.5 will respond to physiological changes in the
photosynthetic biochemistry of a cell (i.e. the light-dependent reactions). The
upper limit of the curve (P max ) will respond to changes in environmental
parameters, such as nutrient concentration and temperature, which affect the
dark reactions of photosynthesis. Because different species of phytoplankton
respond differently to changes in surface radiation and in sita light intensity,
changing environmental conditions will favour different species at different
times and lead to a succession of different dominant species in the
community. Valúes for P max and AP/A/ are given in Table 3.2. Note that
Pmax is generally increased at higher temperatures and under high nutrient
conditions, but the initial slope of the photosynthetic curve (AP/A/) is more
dependent on cellular properties; e.g. picoplankton generally have higher
AP/A/ valúes than larger phytoplankton. Consequently, picoplankton can
grow deeper in the water column where there is less light.

Table 3.2 Representative valúes of P max and AP/A/. AP/A/ is the initial
slope of the curve in Figure 3.5 and is given in terms of productivity divided
by solar radiation; P max is given as the máximum valué of the assimilation
Índex (see Section 3.2.1).

Pmax (assimilation index)

(mg C mg _l Chl a h _l )

Comment

2-14

General range

2-3.5

Low temperatures, 2-4°C

6-10

High temperatures, 8-18°C

0.2-1.0

Low nutrients (e.g. in the
Kuroshio current)

9-17

High nutrients and high
temperatures (e.g. in
tropical Coastal waters)

AP/A/ (initial slope)

(mg C mg -1 Chl a h~ l )/(¡xE m -2 s _1 )

Comment

0.01-0.02

Températe ocean

0.005-0.01

Subtropical waters

0.02-0.06

Picoplankton (< 1.0 fx m)

0.006-0.13

Annual range for températe

(annual average, 0.045)

Coastal waters

QUESTION 3.3 Using equation 3,2 and assuming a valué of
2mgC mg l Chl a h 3 and a valué of 10 gE m" 2 s^ 1 for one species
of phytoplankton and respective P m4X and valúes of
6 mg C mg _1 Chl a h“ ] and 20 juE s H for a second speeies, which
species will be growing faster at a PAR light intensity of 50 /xE m : s^ 1 ?

In an earlier section ( 2 . 1 . 2 ), we considered how to calcúlate the extinction of
light in water and the compensation depth of light. Equations 2.1 and 2.2
can now be extended to deal with the problem of phytoplankton being mixed
vertically in the water column. When phytoplankton are being mixed up and
down in the surface layers of the sea, it is useful to know the average
amount of light (¡a) in the euphotic zone. This is given by the expression:

l D = ^:(\-e- kD ) (3.4)

where 7 q is the surface radiation, k is the extinction coefficient, and D is the
depth over which the light intensity is averaged.

A useful application of equation 3.4 is to consider how far down a
population of phytoplankton cells can be mixed until photosynthetic gain is
balanced by respiratory losses (i.e. where P w = R w , Figure 3.6). This depth
is called the critical depth (Z) cr ). If equation 3.4 is rearranged and le, the
compensation light intensity, is substituted for 7 o, we get the following
expression to calcúlate the critical depth:

¿>cr - ~-(l - e~ kD «)

klc

(3.5)

If kD cr >> 0, then equation 3.5 can be simplified to:

D Qr = (3.6)

klc

The importance of the critical depth is illustrated in Figure 3.6. This figure
indicates that if the amount of plant material used up in respiration (in the
area bounded by ABCD) is matched against the amount gained by
photosynthesis (area ACE), then diagrammatically one arrives at the same
depth as calculated in equation 3.6, that is, the critical depth. If
phytoplankton cells are mixed downward below this depth by intensive
storm action, there can be no net photosynthesis. However, as long as the
depth of mixing is above the critical depth, positive net photosynthesis can
occur. Thus by using a simple formula based on the amount of radiation at
the surface (7 0 ), the extinction coefficient ( k ), and a known compensation
light intensity (7c), it is possible to estimate when the spring production of
phytoplankton can start in températe latitudes.

QUESTION 3.4 The surface radiation is 500 ¡t E m -2 seo -1 of which 5(Wc is
PAR. the compensation üght intensity of the phytoplankton is
10 E m -2 sec -1 , and the depth of mixing in the water column is 100 m.
Using the extinction coefficient obtained in Question 2.2, is there any net
positive photosynthesis ín this water column?

Figure 3.6 An illustration of the relationships among the compensation light depth, the
critical depth, and the depth of mixing. At the compensation depth (De), the light intensity
(le) is such that the photosynthesis of a single cell (Pe) is equa) to its respiration (R c )\
above this depth there is a net gain from photosynthesis (P c > fíe) and below it there is a
net loss (Pc < Re)- As phytoplankton celis are mixed above and below the compensation
depth, they experience an average light intensity (f D ) in the water column. The depth at
which l D equals le is the critical depth (D cr ) where photosynthesis throughout the water
column (P w ) equals phytoplankton respiration throughout the water column (R w ). The area
bounded by points A, B, C and D represents phytoplankton respiration, and the area
bounded by points A, C and E represents photosynthesis; these two areas are equal at the
critical depth. When the critical depth is less than the depth of mixing (Dm) (as illustrated in
this figure), no net production takes place because P w < fí w . Net production of the
phytoplankton (P w > R w ) only occurs when the critical depth lies below the depth of mixing.

3.4 THE EFFECT OF NUTRIENTS ON GROWTH RATE

In Section 3.2.1, productivity was represented as the amount of carbón fixed
per unit time. This is a convenient convention because it is what the
ecologist actually measures. It was also pointed out that productivity can be
represented by the assimilation Índex, in which growth is expressed as mg of
carbón produced per mg of chlorophyll a per hour. This valué is useful for

comparing photosynthesis from different areas because it normalizes all
measurements to a unit of chlorophyll a .

QUESTIONi 3.5 In comps iring two difieren! areas of the ocean, we tind (hal the
phoiosynthetic production is 20 mg C ni 3 h ~ l in area A and
50 mg Cm‘ 3 h 1 ín area B, The standing stocks of phytoplankton are
2 mg Chl a m 1 and 25 mg Chl a m 3 in areas A and B, respectively.

(a) In which area are !he phytoplankton most photosynthetícally active? (Use
assimilation indices to determine the answer.) (h) What could cause this
difieren ce in aclivity?

Another useful way of comparing growth rates of phytoplankton is to
express growth as an increase in cell numbers. For unicellular organisms,
this is an exponential function:

(X 0 + AX) = X 0 ^ (3.7)

where X 0 is the population of cells at the beginning of the experiment, AX
is the number produced during time í, and /x is the growth constant of the
population per unit of time. If AX has been measured in units of
photosynthetic carbón, then Xq musí be expressed as the total standing stock
of phytoplankton carbón instead of in terms of cell numbers.

One additional expression that can be obtained from equation 3.7 is the
doubling time, which is defined as the time taken for a population to
increase by 100%. Doubling times for phytoplankton can be derived from:

X, = X 0 e* lt (3.8)

where X, is (X 0 + AX) in equation 3.7. The time required for Xo to double
(d) is given as,

and then doubling time (d) can be calculated from:

i = log e 2 _ 0.69
{l /x

(3.9)

(3.10)

The reciprocal of doubling time (or 1 /d when d is in days) gives the
generation time as number of generations produced per day.

The effect of nutrient concentration on the growth constant, /x, can be
described by the same expression that was used for photosynthesis
(equations 3.2 and 3.3). Henee,

Mmax [N]

K n + m

(3.11)

where ¡i is the growth rate (time -1 ) at a specific nutrient concentration [N]
which is usually expressed in micromoles (/xM) per litre, /x max is the
máximum growth rate of the phytoplankton, and K N (given in /xM) is a
half-saturation constant for nutrient uptake that is equal to the concentration
of nutrients at 1/2 /x ma x-

Equation 3.11 is valid when the growth rate of phytoplankton is controlled
by the nutrient concentration in seawater. However, in some surface waters

[N]

Iim!

[N]

Figure 37 Three possible variations (a, b, and
c) in the nutrient—growth curves of competing
pairs (species 1 and 2) of hypotheticaí species
of phytoplankton. ¡s the specific growth rate;
/¿ max is the máximum rate of growth; K N is a
half-saturation constant for nutrient uptake; [A/|
is ambient nutrient concentraron; SI is species
1; and S2 is species 2. All units are arbitrary.
See text for discussion of the differences in
resuits between competing species.

with extremely low concentrations of nutrients, some larger photosynthetic
dinoflagellates (See Section 3.1.2) can migrate to deeper layers where
nutrients are more abundant. The zone where nutrient concentrations
increase rapidly with depth is the nutricline, and this may be below the
euphotic zone. After taking nutrients such as nitrate into the cell, these
flagellates can return to sunlit waters to carry out photosynthesis. In such
cases, the (photosynthetic) growth rate of the phytoplankton is proportional
to the nutrients within the cell, and not to the external nutrient concentration.

Among the principal nutrients in the sea that are required for phytoplankton
growth, only certain elements may be in short supply. In general, the
quantities of magnesium, calcium, potassium, sodium, sulphate, chloride, etc.
(Table 2.1) are all in sufficient quantities for plant growth. Carbón dioxide,
which may be limiting in lake waters, is present in excessive quantities in
seawater. However, some essential inorganic substances, like nitrate,
phosphate, silicate, iron, and manganese, may be present in concentrations
that are low enough to be limiting to plant production. There may also be
synergistic effects between essential nutrients. For example, the
concentration of iron in a metabolizable form governs the ability of
phytoplankton to utilize inorganic nitrogen. This is because iron is required
in the enzymes nitrite reducíase and nitrate reducíase, and these enzymes are
necessary for the reduction of nitrite and nitrate to ammonium, which is used
to make amino acids. Large diatoms may be affected by iron limitation, but
small flagellates usually are not because they can take up iron at lower
concentrations. Ocean areas that are limited by iron are characterized by
having high nitrate but low chlorophyll concentrations, and they are referred
to as HNLC areas. They inelude the subarctic North Pacific, Equatorial
Pacific, and parts of the Antarctic Ocean. In addition, certain organic
substances (e.g. vitamin B 12 , thiamine, and biotin) are required for
auxotrophic growth of some phytoplankton, and these substances may also
be in short supply in seawater and thus limiting to growth.

Many different species of phytoplankton can be found living in the same
water mass. Whut fació rs al low the coexistence of so many species, all of
which have the same hasje requi reme rus for sunlight carbón dioxide* and
nutrients. and all of which may compete for requi re me nts that are ¡n limited
supply?

Each phytoplankton species has a specific half-saturation concentration {K N
in equation 3.11) for the uptake of each of the limiting nutrients, and each
species has a different máximum growth rate (/¿ max ). These species-specific
differences in growth rates and responses to nutrients allow a great variety of
phytoplankton to grow in what seems to be a very uniform environment.

This is illustrated in part by Figure 3.7 which shows changes in growth rates
(/x) of different hypotheticaí species of phytoplankton having different valúes
of K n and ¿t max and responding to variations in the ambient concentration of
one nutrient (see equation 3.11). In the first example (a) of this figure,
species 1 has a higher máximum growth rate than species 2. Because Kn is
the same for both species, they grow at the same rate to a certain level of
nutrient, beyond which species 1 continúes to its higher máximum growth
rate. In a second example (b), two different species have the same valué for
¡i max , but they achieve this at different nutrient concentrations. The valué of
Kn is lower for species 1, so it reaches the máximum growth rate at a lower
nutrient concentration. In the last example (c), two other competing species
have differing valúes for both ¿x max and K N , and the competitive advantage

Figure 3.8 Coexistence of phytoplankton
species when limited by two resources. (a) One
species (1) which ¡s limited only by the two
lower concentrations of each resource. (b) Two
species (1 and 2) showing the región of
coexistence on each rate-limiting resource.

(c) Four species showing regions of coexistence
based on each resource: 1 is the best
competitor for resource 1; 2 is the second best;
3, third; and 4, fourth; the competitive rank
order for resource ii is reversed. A circled
number indicates that only that species can
exist under the given circumstances.

shifts between the species as the nutrient concentration changes. At lower
nutrient concentrations, species 2 dominates because it grows faster; but at
higher nutrient concentrations, species 1 becomes dominant because it
achieves a higher máximum rate of growth.

If one considers further that two, three, or many growth-rate limiting
nutrients may occur in any body of water, and that there are also differences
in light and other physical properties such as temperature and salinity, it is
obvious that there is a constantly changing mosaic of rate-limiting factors
governing the growth of phytoplankton. Since each species responds
differently to the mosaic, and since growth cannot be limited by more than
one process at any one time, the physical/chemical restrictions on
phytoplankton growth allow for the coexistence of many species in the same
body of water, with successive changes in the relative abundance of the
component species.

Figure 3.8 further explores how several nutrients and several species can
interact to produce the diverse phytoplankton populations discussed above.

In Figure 3.8a, a single species (1) of phytoplankton is considered in relation
to two potentially limiting resources (e.g. nutrients such as nitrate and
phosphate). The species requires a certain minimum concentration of each
nutrient ( R \* and Rn* valúes). If the concentrations drop below these levels,
species 1 cannot exist even without competition from another species; above
these minimal nutrient levels, species 1 can survive and grow. If a second

(a)

)

without competition:
species 1 persists

without competition:

1 species leliminated

1__

' resource 1 mareases —►

(b)

(c)

phytoplankton species (2) having different nutrient-concentration requirements
is introduced (Figure 3.8b), the situation becomes more complex. In
this hypothetical example, each species can be limited by a different nutrient
concentration. Species 1 is a superior competitor for resource 1; it will
be the only species capable of existing at very low levels of resource 1, and
it will outcompete and exelude species 2 at slightly higher concentrations
of this resource. Conversely, species 2 is the superior member and only
survivor when resource 2 is in low concentrations. Above these minimal
nutrient levels, there is a región where both species can coexist. As more
species, each with its own nutrient requirements, are added to the community
(Figure 3.8c), there are more possibilities of establishing coexistence.

With reference to the examples in Figure 3.8, if both nutrients are abundant
and phytoplankton growth is not limited by any of the K¡^¡ valúes for nutrient
uptake, then species dominance will be determined by the /x max valúes of the
species and the fastest growing phytoplankton species will dominate. In the
extreme case of very high nutrient concentrations, there will be a single
species, the one with the highest ax . Thus at both extremes of the nutrient
field, very low and very high nutrient levels tend to lead to a low diversity
of phytoplankton species. Very low nutrient concentrations can lead to
dominance of the community by a single species with the lowest K N , and
very high nutrient conditions can lead to dominance by a single species
having the highest /x max . If Ihe simple illustrations in Figure 3.8 are
expanded to inelude additional resources and other species, an almost infinite
combination of physical/chemical backgrounds is produced in which many
phytoplankton species can grow.

The physico-chemical environmental mosaic itself is not constant. The light
and temperature background changes daily and seasonally, and nutrient
concentrations vary. Sometimes it is the change itself that affeets different
phytoplankton responses. For example, nutrient concentrations may change
sporadically by pulsing inputs resulting from diel upwelling of deeper water
with high nutrient levels. Such fluctuadng changes in nutrient concentration
will have a different effect on phytoplankton species composition than when
nutrients are maintained at relatively constant levels through, for example,
sustained upwelling. On the other hand, toxic pollutants will work in the
opposite direction to nutrient resources; at higher concentrations, they will
selectively inhibit the growth of certain phytoplankton species, so that
eventually diversity is reduced to only the most pollutant-resistant forms. It
must also be added that selective grazing by herbivorous zooplankton can
alter the relative abundance of phytoplankton species.

Some valúes for growth rates (/x max ) and half-saturation constants (Kn) for
phytoplankton are given in Table 3.3. The relative availability of nutrients
for phytoplankton (particularly of nitrate and ammonium which are most
often present in limiting quantities) can be used to classify aquatic
environments. Regions that have low concentrations of essential nutrients,
and therefore low primary productivity, are called oligotrophic. Such areas
typically have chlorophyll concentrations ranging from <0.05 gg l' 1 at the
surface to a máximum of 0.1-0.5 gg l -1 at depths of 100-150 m.

Eutrophic waters contain nutrients in high concentrations; high
phytoplankton densities are manifested by chlorophyll concentrations of 1 to
10 gg 1 _I in surface layers. Mesotrophic is a term that is sometimes applied
to waters of intermediate character. Eutrophic waters tend to be dominated
by one or two fast-growing, r-selected phytoplankton species (see
Table 1.1). In contrast, oligotrophic waters tend to have many competing

Table 3.3 Máximum growth rates (¿i max ) and half-saturation constants (K N ) for
some phytoplankton.

Máximum growth rates (¡i max )
(in generations day~ l )

Comments

0.1-0.2

Oligotrophic, tropical waters

0.4-LO

Températe, eutrophic Coastal

waters

1.0-3.0

Tropical upwelling; and
picoplankton under
eutrophic conditions
and high temperatures

Half-saturation constants ( Kv)
(in /xM)

Nitrate or ammonium

0.01-0.1

Oligotrophic waters

0.5-2.0

Eutrophic oceanic waters

2.0-10.0

Eutrophic Coastal waters

Silicate

0.5-5.0

General range for diatoms

Phosphate

0.02-0.5

General range for olig¬
otrophic to eutrophic waters

/c-selected species, each dependent on a different limiting nutrient; the
community thus tends to be in equilibrium with the total nutrient supply.

QUESTIQN 3.6 From Table 3.3. what is the general relatíonshíp between the
definítions oí eutrophic and oligotrophic and the half-saturation constants
{K¡y) of phytoplankton celia? What dees ibis itnply in rerms of relative
nutrient uptake bv the phytoplankton living in eutrophic or oligotrophic
waters?

3.5 PHYSICAL CONTROLS OF PRIMARY PR0DUCT10N

Light is one of the two major physical factors controlling phytoplankton
production in the sea. The second ineludes those physical forces which bring
nutrients up from deep water, where they accumulate, into the euphotic zone.
These two features together largely determine what type of phytoplankton
develop and how much primary production occurs in any part of the world’s
ocean. They are also major factors in determining the amount and type of
marine animáis that are produced, including fish which are caught
commercially.

The amount of light decreases from the Equator towards the poles. On the
other hand, the amount of wind mixing, which brings nutrients up to the
surface, increases from the tropics (where water is vertically stabilized by
solar heating) toward the poles. Thus the abundance of light and the
abundance of nutrients in the euphotic zone form an inverse relationship
(Figure 3.9) which largely determines the pattern of phytoplankton
production in different latitudes. In polar regions, a single pulse of
phytoplankton abundance occurs during the summer when light becomes
sufficient for a net increase in primary productivity. In températe latitudes,

primary productivity is generally maximal in the spring and again in the
autumn when the combination of available light and high nutrient
concentrations allows plankton blooms to occur. In the tropics, where intense
surface heating produces a permanent thermocline (see Section 2.2.2), the
phytoplankton are generally nutrient-limited throughout the year, and there
are only small and irregular fluctuations in primary production due to local
conditions.

Figure 3.9 is a general representation of the annual cycle of phytoplankton
production in the world’s ocean. However, there are many physical features
that affect nutrient levels in the euphotic zone and thereby greatly modify
the general pattern. These inelude fronts, which are relatively narrow
regions characterized by large horizontal gradients in variables such as
temperature, salinity, and density, and eddy-formations such as rings and
large-scale gyres, which have characteristic rotational patterns of circulation.
These modifying physical features may be thousands of kilometres wide
(e.g. gyres) or only a few kilometres long (e.g. tidal and river-plume fronts).
The size depends on the topography and ocean climate of any particular

Figure 3.9 The relative abundance of light (unshaded area) and nutrients (shaded area) at
the sea surface and the relative seasonal change in primary productivity at three different
latitudes. (Productivity expressed in arbitrary vertical scales.)

-O

r'-.-.v

Températe productivity

Tropical productivity

Winter

Spring

Summer

Autumn

Winter

location. The common feature of all these structures is that there is some
mechanism involved for bringing nutrients up to the euphotic zone from
deeper water, on time scales which may range from days to months. These
mechanisms are superimposed on the seasonal wind mixing that partly
generates the global pattern of phytoplankton production shown
schematically in Figure 3.9. Some of the nutrient-enhancing processes can
result in ‘oases’ of plankton production during periods of the year when the
production of phytoplankton would otherwise be low.

3.5.1 OCEANIC GYRES AND RINGS

The general circulation of surface water in the global ocean (discussed in
Section 2.6 and shown in Figures 2.19 and 3.10) results in large gyres. In
the anticyclonic gyres, water flows in a clockwise direction in the Northern
Hemisphere, and in an anticlockwise direction in the Southern Hemisphere
(see Table 3.4). In the Northern Hemisphere, the clockwise flow results in
convergent gyres because the direction of water circulation tends to draw
surface water in toward the centre. This is illustrated in Figure 3.11b where
it can be seen that anticyclonic gyres in the Northern Hemisphere tend to
deepen the thermocline due to the convergent tendency of the circulation. In
this situation, no new nutrients can come to the surface from the deep water,

Figure 3.10 The location of upwelling zones and coral reefs in the world’s ocean.

Figure 3.11 Plan and cross-sectional views of
a cyclonic (a) and aníicyclonic (b) gyre in the
Northern Hemisphere. The dashed arrows
indícate net transport of water away from and
towards the centre, respectively. The same
pattern of circulation applies to warm core and
coid core rings, but on a smaller scale.

Table 3.4 Water flow in gyres and rings in the Northern and Southern Hemispheres.

Cyclonic gyres

Anticyclonic gyres

Coid core rings

Warm core rings

divergence

convergence

leading to

high production

low production

Northern

Hemisphere

Southern

Hemisphere

and convergent gyres like the Sargasso Sea in the North Atlantic are relative
‘deserts’ of ocean production. In the Southern Hemisphere, the reverse
rotational direction of the gyres also reverses the vertical flow of water
within the system, so that the anticlockwise circulation also forms
convergent gyres with relatively low productivity.

Cyclonic gyres are formed by water circulating in an anticlockwise direction
in the Northern Hemisphere and in a clockwise direction in the Southern
Hemisphere. These are divergent gyres, which tend to draw water up from
below the thermocline (Figure 3.11a); this results in a plentiful supply of
nutrients at the surface that should make such areas highly productive. The
Alaskan Gyre in the Gulf of Alaska is a divergent gyre in which the actual
vertical movement of water from below the thermocline is believed to be
about 10 m yr _I . Although this would be a highly productive gyre if situated
farther south, its location at north of 50° N means that the area is limited by
light in winter, and the productivity of the gyre is actually controlled more
by seasonal events than by oceanic circulation.

gyre gyre

upwelling

divergence

no upweliíng

convergen ce

direction oí
current flow

(b)

Figure 3.12 The sequential formation of
warm (W) and coid (C) core rings from a major
current system (such as the Gulf Stream) in the
Northern Hemisphere. As the current flows
between water of contrasting temperature, it
begins to develop a meandering pattern (a) with
eddies forming on the edges. As the meanders
become more pronounced (b), the eddies are
eventually pinched off to form independent
circulatory Systems called rings (c). Note that
this results in warm water rings being isolated
in areas of coid water, and in coid water being
transferred across the current into an area of
predominantly warm temperatures.

Ring structure in the ocean have the same morphology as gyres, but they are
much smaller, being hundreds, rather than thousands, of kilometres in
diameter. They are formed as eddies that detach from a major current system
such as the Gulf Stream. Such large currents tend to meander and, in so
doing, large eddies or rings spin off as independent bodies of circulating
water which may survive for several years (i.e. long enough to influence the
primary productivity within the ring). The two types of rings shown in
Figures 3.12 and Colour Píate 7 are referred to as warm core rings
(anticyclonic) and coid core rings (cyclonic). A cross-section of each ring
type would look like the cyclonic and anticyclonic gyres shown in
Figure 3.11, but on a much smaller scale. The rotational circulation of the
rings maintains cooler (cyclonic circulation) or warmer (anticyclonic)
temperatures because of the respective vertical flow of water within the rings
(see Table 3.4 and Figure 3.11). However, although the isotherms in coid
core rings bow up in the middle as they do in cyclonic gyres (Figure 3.11),
this does not necessarily signify upwelling. High productivity within coid
core rings may result because the water which has been captured by the
meander is already nutrient-rich. Similarly, water in the centre of warm core
rings is not necessarily sinking.

3.5.2 CONTINENTAL CONVERGENCE AND DIVERGENCE

Very large frontal zones occur along the edges of continents due to
wind-driven oceanic circulation. Major divergent continental fronts are
associated with the Perú Current and California Current in the Pacific, and
with the Canaries Current and Benguela Current in the Atlantic
(Figure 2.19). Currents such as these that flow toward the Equator along the
western coasts of continents are driven away from the coasts due to the
EartlTs eastward rotation, and this consequently leads to Coastal upwelling
(Figure 3.10). Upwelling of nutrient-rich water in these areas continúes for
many months of the year. Further, the location of these currents in latitudes
between 10° and 40° means there is generally enough solar radiation to
allow máximum photosynthesis during most of the year. These four
divergent continental fronts are among the most productive regions in the
ocean. They are characterized by having large populations of fish and birds,
and they have been the subject of much scientific investigation because of
their exploitable resources.

Another divergent continental front exhibiting upwelling and extremely high
production occurs around the continent of Antárctica. Known as the
Antarctic Divergence, this area is the home of huge stocks of krill and
other zooplankton which give rise to abundant stocks of whales, seáis, and
seabirds (see Section 5.2 and Figure 5.4).

Contrary to expectations, the west coast of Australia does not support a large
fishery that would be indicative of upwelling. Although water does have a
tendency to upwell on this western coast, the upwelling is suppressed by a
continual strong flow of warm water from the north which covers the area
(Figure 3.10). A similar flow of warm water across the Pacific Ocean can
sometimes suppress the effect of the Peruvian upwelling by greatly
increasing the depth of the thermo-nutricline, an event that has become
known as an El Niño.

An opposite type of convergent continental front tends to form on the
eastern sides of continents, where water flows away from the Equator. These

regions are characterized by the accumulation of large quantities of warm,
nutrient-poor water. They are usually areas where coral reefs occur in
máximum abundance (Figure 3.10); these inelude the Great Barrier Reef off
eastern Australia in the South Pacific, the coral reefs of Madagascar in the
Indian Ocean, and the reefs of the Caribbean Sea.

3.5.3 PLANETARY FRONTAL SYSTEMS

The continental frontal systems described above are large enough to be
described as planetary fronts, but they have been dealt with separately
because of their very special association with continents. Other planetary
frontal systems are formed by the convergence or divergence of two current
systems which often have contrasting properties. Thus the Oyashio off the
northern coast of Japan is a coid nutrient-rich current that meets the warm
and vertically stable Kuroshio in the western Pacific (Figure 2.19). These
two currents join to form the North Pacific Current which flows from Japan
to the west coast of North America. Mixing of these waters produces a very
large frontal zone that is highly productive for marine life. A similar
situation occurs in the North Atlantic where the coid Labrador Current meets
the warm Gulf Stream (Figures 2.19 and Colour Píate 7).

A planetary frontal system is also formed around the Antarctic continent at
latitudes of about 57°-59°S; here there is a convergence of subtropical water
with Antarctic water, forming the Antarctic Polar Front (or Antarctic
Convergence). This convergent zone of sinking water is an important source
of coid deep water for the world’s ocean (see Section 2.4 and Figure 2.17).

Finally, the last of the fronts which can affect productivity on a planetary
scale is the upwelling caused largely by divergent current patterns across the
Equator. Equatorial upwelling is particularly pronounced at about 10°N in
the Pacific Ocean, where it results in an extensión of the Californian and
Peruvian continental upwellings out into the Pacific Ocean. It also occurs in
the Atlantic and to a lesser extent in the Indian Ocean.

3.5.4 SHELF-BREAK FRONTS

Shelf-break fronts occur along the edges of continental shelves and other
banks (which are often undersea ‘island’ extensions of the continental
shelves). A shelf-break front is formed by a combination of the sudden
shallowing of water across a continental shelf, and by the change in current
speed across the shelf which may be induced by residual oceanic circulation
or, especially, by tidal exchange. The process by which a shelf-break front is
formed can be analysed by considering the ratio (R) of the potential energy
(PE) in maintaining well-mixed conditions to the rate of current energy
dissipation ( TED ) in a water column of unit cross-sectional area:

R =

TED

(3.12)

The two forms of energy (PE and TED) can be formulated in terms of a
number of parameters, most of which are constant for a defined area where
the major form of stratification is a thermocline. Two important terms which
are not constant are the average water velocity, \U\, and water depth, h.
These are considered in formulating a stratification Índex expressed as:

-=-(in c.g.s. umts)

Cz>|t /| 3

5 = logio

(3.13)

Figure 3.13 (a) Average stratification índex
valúes obtained for the Celtic Sea between
Ireland and England.

(b) Surface distribution of chlorophyll a, in
April, for the Celtic Sea.

where C& is a frictional or drag coefficient that can be approximated as a
constant ( ca . 0.003) for a sandy bottom. The stratification Índex can be
easily calculated for any Coastal región, and it usually falls within the range
of + 3and — 2, the former valué indicating highly stratified water and the
latter, highly turbulent, A valué of S ^ 1.5 provides the best conditions for
phytoplankton production, indicating water that is not too stratified and not
too turbulent. Nutrients that are brought to the surface by turbulence as the
water velocity increases over a shelf or bank can be utilized by the
phytoplankton, resulting in a shelf-break front of high primary productivity.
Note, however, that the highest standing stock of phytoplankton (i.e.

dislance (nautical miles)

Figure 3.14 (a) Chlorophyll a valúes through a frontal región.

(b) Corresponding temperature (°C) through a frontal región.

chlorophyll a concentration) will develop over time on the more stable side
of the front.

Shelf-break fronts are illustrated in Figures 3.13 and 3.14. Calculations of
the stratification Índex (Figure 3.13a) coincide spatially with a chlorophyll
máximum (Figure 3.13b) in the Celtic Sea. In Figure 3.14, the vertical
distribution of chlorophyll is shown within a mixed water column on one
side of the front and within a stable water column on the other side of the
front.

CUESTION 3.7 The mean tidal flow across a shallow bank havíng a mínimum
depth of 50 m is 3.3 cm s _l T Assuming a sandy bollom. will ibis bank
produce a frontal /one?

3.5.5 RIVER-PLUME FRONTS

Rivers entering the sea often carry high nutrients, derived either from natural
sources or from agricultural fertilizers and sewage. These nutrients enrich

(a)

Figure 3.15 Nutrient enlrainment at the mouth
of a river (a) Cross-sectional view. (b) Plan
view.

River plume
of

brackish water

Coastal waters and increase productivity off the mouth of the river. In
addition, estuarine waters are often highly productive because the flow of the
river at the sea surface causes nutrients to be entrained from deeper water
(Figure 3.15) upwelling into the surface water. Providing the deep waters are
rich in phosphates and nitrates, the entrainment of nutrients also contributes
to phytoplankton blooms off the river mouth. The exact position of a bloom
in the river plume (or the location of the front) is a function of many factors
including the quantity of nutrients introduced and/or entrained, the settling
out of river silt which allows light to penétrate deeper, the depth of the
mixed layer, grazing by zooplankton, etc. A phytoplankton bloom may also
be disrupted or enhanced by the prevailing oceanic climate affecting the
estuary.

3.5.6 ISLAND MASS EFFECT AND LANGMUIR FRONTAL ZONES

In addition to the five major physical processes that bring nutrients up to the
euphotic zone as discussed above (Sections 3.5.1-3.5.5), there are many
additional minor effects that form smaller frontal zones by physically
altering nutrient concentrations in surface waters. Among these is the island
mass effect (also known as island wake effect). This is a disturbance in the
flow of water caused by the presence of an island (or an undersea mountain),
resulting in upwelling from below the thermocline and subsequent nutrient
enrichment of surface waters. First described from enhanced phytoplankton
biomass and production around Hawaii, it is now known in many localities.
For example, a plume of high production (> 4 mg Chl a m~ 3 ) extends west
of the Scilly Isles (off south-west England) for about 50 km into water that
otherwise contains less than 0.5 mg Chl a m” 3 . Similar upwelling and
enhanced production can result as currents pass headlands and bays on a
rugged coastline.

A different process affecting production on a smaller scale is Langmuir
circulation. This pattern of circulation is set up when wind blows steadily
across the surface of relatively calm seas. As a result, vórtices of several

Figure 3.16 Langmuir vórtices and plankton
distributions. Neutrally buoyant partióles are
randomly distributed, but downward swimming
organisms are aggregated in high velocity
upwellings (A); partióles that tend to float are
aggregated in downwellings (B); partióles that
tend to sink are aggregated in upwellings (C);
upward swimming organisms are aggregated in
low-velocity downwellings (D); and horizontally
swimming organisms are aggregated where
there is less relative current velocity than within
the vórtices (E).

metres in diameter start to revolve around horizontal axes and lead to both
upwelling and downwelling of water (Figure 3.16). The vertical scale of this
interaction is not large enough to bring nutrients up from deep water, but it
is sufficient to concéntrate plankton and this will enhance feeding
interactions and result in a faster regeneration of nutrients. The phenomenon
of Langmuir circulation is often visibly apparent as a series of parallel foam
lines extending for great distances. In the Sargasso Sea, the seaweed
Sargassum lines up in windrows in response to this type of circulation.

3.6 GLOBAL PHYTOPLANKTON PRODUCTIVITY

The primary productivity of phytoplankton in various areas of the global
ocean varíes with season and location. The highest productivity valúes of
>1 g C nrT 2 day -1 are encountered in upwelling areas (see Section 3.5.2),
and the lowest valúes (<0.1 g C m -2 day -1 ) occur in the subtropical
convergent gyres (see Section 3.5.1). During the summer in subarctic
latitudes of the Pacific and Atlantic oceans, daily primary productivity may
be >0.5 g C m -2 , but during the winter there may be no net primary
productivity for several months. Integration of these different valúes on an
annual basis gives the range of primary productivity valúes shown in
Table 3.5. These differences in relative production can also be seen in
Colour Píate 8 which shows relative chlorophyll concentrations in surface
waters of the global ocean as detected by remóte sensing from satellites. In
total, the primary productivity of the world’s ocean is about 40 x 10 9 tonnes
of carbón per year. This figure is the same order of magnitude as for
photosynthetic production by terrestrial plants, but the pattern of production
is very different.

QUESTIQN 3.8 Much of the ludían Ocean between latitudes 0 and 40 S has
a low primary productivity of less than 150 mg C m -2 day -1 . What
feaíure(s) limit production in this ocean?

In the terrestrial ecosystem, very high productivities occur in relatively small
areas and ■ -e range of valúes for production are very great. For example, the

Table 3.5 The range of annual primary productivity in different regions of the
global ocean.

Location

Mean annual primary productivity
(g C m -2 year -1 )

Continental upwelling

500-600

(e.g. Perú Current,

Benguela Current)

Continental shelf-breaks

300-500

(e.g. European shelf, Grand

Banks, Patagonia shelf)

Subarctic oceans

150-300

(e.g. North Atlantic,

North Pacific)

Anticyclonic gyres

50-150

(e.g. Sargasso Sea,
subtropical Pacific)

Arctic Ocean (ice-covered)

<50

estimated primary productivity of a rainforest is 3500 g C m -2 year -1 , or
about six times the highest phytoplankton productivity. On the other hand,
much of the terrestrial land mass is desert with little or no photosynthetic
production. In contrast, marine productivity occurs virtually everywhere in
the euphotic zone of the oceans (covering > 70% of the planet’s surface),
even under polar ice. It is the accumulative effect of the marine primary
productivity throughout the world ocean that adds up to a total annual
production of photosynthetic carbón approximately equivalent to that on
land.

Latitudinal and seasonal differences in marine productivity result from
differences in light and nutrient availability (see Figure 3.9). These physical
forces largely determine the maximal phytoplankton production that is
possible in any marine area. There are also biological processes that modify
regional primary production levels. As algae grow, they reduce nutrient
concentrations in the euphotic zone, and their own increasing numbers create
self-shading by reducing the penetration of light, thus causing the euphotic
zone to become shallower. Balancing these effects are the grazing activities
of herbivorous zooplankton which remove part of the production, and there
are regional differences in how the phytoplankton community is utilized by
these animáis.

When primary productivity increases, it is generally accompanied by a
measurable increase in the standing stock of phytoplankton. During a bloom
in coastal areas, the standing stock of chlorophyll a may increase from less
than 1 mg m -3 to more than 20 mg m -3 over a period of several days. In
some areas, however, the zooplankton may graze the phytoplankton as fast
as it is produced, with the result that the increase in primary productivity
does not show any discernible increase in the standing stock of
phytoplankton. This situation is found in the North Pacific Ocean at about
50° N (Figure 3.17). Here, outside of coastal influences, there is virtually no
change in the standing stock of phytoplankton throughout the year; it remains
constant at about 0.5 mg chlorophyll a m -3 . However, primary productivity
in this area increases from winter valúes of less than 50 mg C m -2 day' 1
to more than 250 mg C m ' 2 day -1 in July. The excess primary productivity
is grazed by the indigenous zooplankton which increase their biomass as

JFMAMJ JASOND J FMAMJJ ASON D

North Pacific ^ Tropical

JFMAMJ JASOND J FMAMJ JASOND

Figure 3.17 Summary of annual cycles in plankton communities in different regions. The
solid black lines represent changes in phytoplankton biomass; the dashed blue lines indícate
changes in zooplankton biomass (arbitrary units).

indicated in Figure 3.17. The cióse phasing between phytoplankton and
zooplankton also has implications for deep-sea benthos in the North Pacific
as there is little uneaten phytoplankton sinking into deep water to serve as a
food supply for benthic animáis (see Section 8.8.4).

In contrast, in the Altantic Ocean at the same latitude, the spring bloom is
characterized by a ten-fold increase in chlorophyll a from 0.1 to about
1.0 mg m -3 . Primary productivity increases as in the Pacific Ocean, but the
zooplankton are less efficient at keeping pace with increases in primary
production. Because only a fraction of the phytoplankton are eaten, there is
an increase in the standing stock as measured by chlorophyll a. In much of
the North Atlantic there is also an autumn bloom of phytoplankton, shown in
Figure 3.17 as a second peak in phytoplankton and zooplankton biomass. As
much of the phytoplankton is not eaten in North Atlantic waters, decaying
blooms sink into deep water and become a food source for animáis on the
seafloor.

Two other annual cycles of phytoplankton and zooplankton are shown in
Figure 3.17. One shows the pattern in the Arctic Ocean, where a single pulse
of phytoplankton occurs soon after the disappearance of the ice and is
followed somewhat slowly by a single pulse in the biomass of zooplankton.
The lag in response time of the zooplankton to increased food is due to
relatively slow growth rates in coid water. In tropical environments, the
biomass of phytoplankton and zooplankton shows no substantial change
throughout the year. However, storm activities can disrupt this otherwise
very stable environment so that small pulses in plankton biomass may occur
irregularly throughout the year. In warm tropical water, any increase in
phytoplankton standing stock is quickly tracked by the fast-growing
zooplankton.

Primary productivity varíes with depth, and the vertical distribution of
phytoplankton may change seasonally. This is illustrated over a time

Figure 3.18 Schematic seasonal depth
changes in phytoplankton biomass (S), daily net
photosynthetic rate (P n ), and nutrient
concentration (N) in stratified températe water.

S (shaded area), usually expressed in mg chl a
rcr 3 , P n (broken line), usually expressed as mg
C per mg Chl a per day, N (blue line), usually
expressed as (i M nitrate. The figure omits any
changes caused by significant zooplankton
grazing.

sequence in Figure 3.18. In températe latitudes, phytoplankton will be well
mixed in the surface layer during the winter months and any photosynthesis
will follow a light attenuation curve (Figure 2.5) except for some
photoinhibition near the surface. As the spring progresses, primary
productivity will increase near the surface, and this may be accompanied by
an increase in the standing stock of phytoplankton. In late summer as the
System runs out of nutrients near the surface, máximum primary productivity
will shift deeper in the water column, resulting in a chlorophyll máximum at
depth.

In stable water masses (i.e. most tropical and subtropical oceans), the
vertical distribution of nutrients, primary productivity, and chlorophyll a
resembles that shown for late summer in Figure 3.18 and is characteristic of
the water mass throughout the year. Chlorophyll maxima in such waters can
be found anywhere from 20 m to over 100 m depending on the long-term
stability of the water. Under these conditions, the euphotic zone is really
divided vertically into two communities. The top community is
nutrient-limited and largely govemed by biological and Chemical processes
that regenérate nutrients within the zone. The bottom community is
light-limited, but it is located at the nutricline, where the máximum change
in nutrient concentration occurs, and thus additional nutrients enter the
system from deeper water. Since some zooplankton and fish migrate
vertically through both communities, there is a degree of biological transport
between the two vertically separated environments.

3.7 SUMMARY OF CHAPTER 3

1 The marine phytoplankton community is composed of severa! diverse
groups of algae that carry out autotrophic production and begin the pelagic
marine food chain. Photosynthesis results in the production of high-energy
organic materials from carbón dioxide and water plus inorganic nutrients.

2 Photosynthesis involves a series of interrelated Chemical reactions. The
light reactions depend upon chlorophyll and accessory pigments capturing
photons of light, so that radiant energy is converted to Chemical energy. The
dark reactions do not require light; they reduce the carbón dioxide and
produce high-energy carbohydrates as end producís. Respiration in plants and

animáis is the reverse process of photosynthesis, whereby oxygen is used to
release the energy contained in carbohydrates and carbón dioxide is liberated.

3 All phytoplankton species require certain inorganic substances to carry
out photosynthesis, including sources of nitrogen, phosphorus, and iron (also
silica for diatoms) which may be in concentrations that are low enough to be
limiting to plant production. Some species also require certain organic
substances (e.g. vitamins) for auxotrophic growth, and these also may be
present in limiting concentrations.

4 Estimates of the total phytoplankton crop (standing stock or biomass) in
a particular locality can be determined by measurements of cell numbers,
total volume, or most commonly, by quantity of chlorophyll a. The rate of
primary production is most often measured by following the uptake of
radioactive 14 C in samples of seawater containing phytoplankton.

5 The amount of photosynthesis increases with light intensity up to a
máximum valué known as P max which is specific for each species. When
light intensity increases beyond this valué, the rate of photosynthesis
declines due to photoinhibition. The light intensity at which plant
photosynthesis (production) exactly equals plant respiration (losses) is the
compensation intensity. Gross photosynthesis describes total photosynthesis;
net photosynthesis is gross photosynthesis less respiratory losses.

6 Photosynthetic responses of phytoplankton species to light can be
described by a series of equations based on valúes for P m ax and K¡. P max
valúes are generally higher at warmer temperatures and in eutrophic waters.

7 Phytoplankton are exposed to differing light intensities as light changes
over the course of a day and as the algae are mixed vertically in the surface
layers of the sea. At the critical depth, photosynthetic gains throughout the
water column are just balanced by respiratory losses in the phytoplankton. If
the depth of water mixing is greater than the critical depth, no net primary
production can take place. Net production occurs only when the critical
depth exceeds the depth of mixing.

8 Growth rates of phytoplankton are also controlled by the concentrations
of essential nutrients in seawater. Oligotrophic regions have low
concentrations of essential nutrients and therefore low productivity. Eutrophic
waters contain high nutrients and support high numbers of phytoplankton.

9 Each species of phytoplankton has a particular response to different
concentrations of limiting nutrients, and each has a different máximum
growth rate. These differences and the species-specific responses to different
light intensities, temperatures, salinities and other parameters, mean that
heterogeneous and fluctuating environmental conditions favour different
species at different times and allow many species to coexist in the same
body of water. Thus phytoplankton species diversity can be high in what
appears superficially to be a homogeneous aqueous environment.

10 Solar radiation and essential nutrient availability are the dominant
physical factors controlling phytoplankton production in the sea. The amount
of light varies with latitude, and the amount of nutrients contained in the
euphotic zone is largely determined by physical factors controlling vertical
mixing of water.

11 Despite year-round high light intensity, tropical regions are generally
low in productivity because solar heating stabilizes the water column and

nutrients remain at low concentrations within the euphotic zone. Conversely,
polar regions are generally high in nutrients but low in solar radiation except
for a brief period in the summer. Máximum annual productivities are
generally found in températe latitudes where light and nutrients are both
reasonably abundant.

12 The general latitudinal pattems of primary productivity are altered by a
number of different physical processes that lead to nutrients being
redistributed in the water column in discrete areas. These processes occur on
scales varying from very large (e.g. gyres and continental upwelling), to
smaller (e.g. tidal fronts and rings), to the very small scales of Langmuir
circulation in which only the top few metres of the water column are mixed.

13 The standing stock of phytoplankton in the surface layers of the sea
ranges from less than 1 mg chlorophyll a m -3 to about 20 mg m 3 during a
phytoplankton bloom. Regional oceanic primary productivity ranges from
<50 to >600 gCm" 2 year -1 , with Coastal upwelling regions having the
highest valúes. Total primary productivity of the world ocean is about

40 x 10 9 tonnes of carbón per year, a figure that is approximately equivalent
to terrestrial plant production.

14 Zooplankton grazing removes different proportions of the phytoplankton
production in different marine areas. Much of the plant production is
consumed in areas where growth rates and generation times of the
zooplankton permit tight coupling with any phytoplankton increase (e.g.
tropical waters). Where there is a lag in the development of zooplankton
relative to increases in phytoplankton biomass, then some of the algal
community dies and sinks to become a food source for deeper-living pelagic
or benthic animáis (e.g. North Atlantic).

15 The vertical profile of phytoplankton production changes with season
and with Iatitude. High surface productivities generally occur in températe
latitudes in spring and autumn, whereas chlorophyll and productivity
maxima occur considerably deeper in tropical waters.

Now try the following questions to consolídate your understanding ofthis
Chapter.

QUESTtON 3.9 1 í í s ge ñera 1 1 y coi iside red that a high ti i versi í y o f spec ies i s
found in spatially heterogeneous environments sueh as rainforests and coral
reefs. What are the reasons for the greal diversity of phytoplankton (see
Table 3.1) found in the pelagic en virón merst?

QUESTION 3.10 if a rute of photosynthesis is measured at 0.2 mg C ni h 5
and the standing stock oí phytoplankton is 2.5 mg C m \ what is the
doubhng time of the phytoplankton in the sample? Refer to equations 3.7
and 3.10,

CUESTION 3.11 Refer to Table 3.3 (a) Is the growth rale of the popa kit ion in
Question 3JO rapid or slow? (b) Where might yon tind sueh a population
growing?

QUESTION 3.12 If the half-saturaüon canstants iK\) for ni trate uptake were
0.1 fiM for spec ies A and 0.5 / 1 M for spec ies R and the máximum growth
rates (/¿ nm ) of A and R were I and 2 douhlings per day respedivdy. which
spec i es would domínate at a n ¡trate concentraban of 0.4 /iM? This can besi
be shown by drawing a graph.

QlLESTfüN 3.13 Whieh icrm in the equatíoti (3.1 1 ) for nulrient upiake bv
phytoplankton is more ¡mportant in determining high species diversity* K %
or ¿w?

QUEST1DN 3.14 Couid you control the type of phytoplankton that grow ¡n ilie
sea through the iniroduction of artificial nutrió ni media?

QUESTION 3.15 What íeaiures would aífeet the growth rales and type of
phytoplankton living under ice in the Arcüe Ocean?

QUESTION 3.16 In what ways can phytoplankton produce harmful effeets on
marine animáis and hunuins?

CHAPTER 4

ZOOPLANKTON

Figure 4.1 A ‘Bongo’ zooplankton sampler,
consisting oí duplícate plankton nets, being
retrieved on board a research vessel.

The animáis making up the zooplankton are taxonomically and structurally
diverse. They range in size from microscopio, unicellular organisms to
jellyfish several metres in diameter (refer to Figure 1.2). Although all
zooplankton are capable of movement, by definition none are capable of
making their way against a current. By definition also, all
zooplankton — indeed all animáis and some micro-organisms — are
heterotrophic. That is, they require organic substrates (as opposed to
inorganic ones) as sources of Chemical energy in order to synthesize body
materials. Unlike plants, which carry out autotrophic production by utilizing
solar energy to reduce carbón dioxide, animáis obtain carbón and other
essential Chemicals by ingesting organic materials. Animal species differ in
how their energy is obtained: some species are herbivores which consume
plants; others are carnivores which are capable of eating only other animáis;
and some species are predominantly detritivores which consume dead
organic material. Many animáis, however, are omnivores with mixed diets
of plant and animal material. Different types of zooplankton often are placed
in categories which describe their diets.

In addition to size categories and positions in food chains, zooplankton can
be subdivided into classifications based on habitat (oceanic vs. neritic
species; see Section 1.2) and taxonomy. They also form two categories
depending upon the length of residency in the pelagic environment;
holoplankton (or permanent plankton) spend their entire Ufe cycles in the
water column, whereas meroplankton are temporary residents of the
plankton community. The meroplankton ineludes fish eggs and fish larvae
(the adults are nektonic), as well as the swimming larval stages of many
benthic invertebrates such as clams, snails, barnacles, and starfish. The more
common types of holoplankton and meroplankton are described below in
Sections 4.2 and 4.3, respectively.

4.1 COLLECTION METHODS

Zooplankton larger than 200 /¿m (refer to Figure 1.2) traditionally ha ve been
collected by towing relatively fine-mesh nets through the water column.
Plankton nets vary in size, shape, and mesh size (Figure 4.1), but all are
designed to capture drifting or relatively slow-moving animáis that are
retained by the mesh. The simplest nets are conical in shape, with the wide
mouth opening attached to a metal ring and the narrow tapered end fastened
to a collecting jar known as the cod end. This type of net can vary in length
and diameter and in mesh size, and it can be towed vertically, horizontally,
or obliquely through the desired sampling depths. Such a net will filter water
and collect animáis during an entire towing period. More sophisticated nets
are equipped to be opened and closed at selected depth intervals, and a series
of such nets may be attached to a single frame to allow sampling of different
discrete depths during a single towing operation. Analyses of the collected
samples permit a more detailed picture of the vertical distribution of
zooplankton within a particular area. Because many zooplankton migrate
vertically during each 24-hour period, the time of sampling is also critical in
tracking these changes in depth distribution.

The selection of a particular net depends upon the type of organisms desired
and the characteristics of the water being sampled. For example, a fine-mesh
net (with a mesh opening of ca. 100-200 (i m) obviously must be used to
collect small mesozooplankton. However, the same net is not suitable for
sampling larger, relatively fast-swimming zooplankton, like fish larvae,
because it clogs quickly and must be towed slowly to avoid tearing. ín deep
water, where zooplankton tend to be larger and are less abundant, it is
common to use a very large coarse-mesh net. All nets can be equipped with
a flowmeter that estimates the total volume of water filtered during a tow;
this permits a quantitative representation of the zooplankton collected. The
newest towed samplers, such as the Batfish shown in Figure 4.2,
simultaneously measure salinity, temperature, depth, and chlorophyll a
concentration while counting and sizing zooplankton that pass through an
optical sensor. Such devices can be set to undulate over a set depth range,
thus making it possible to obtain samples at several depths.

No single sampler is capable of capturing all zooplankton within its path.
Zooplankton smaller than 200 ¡im (nano- and microplankton) cannot be
satisfactorily sampled in nets; instead, a known volume of water is collected
in sampling bottles or by pumps from defined depths and the smallest
zooplankton are concentrated and removed by filtration, centrifuging, or
settling and sedimentation. Both planktonic protozoans and phytoplankton
can be counted in the concentrated water samples.

Figure 4.2 A towed Batfish plankton sampler which simultaneously estimates phytoplankton
and zooplankton abundance while recording environmental parameters. F, fluorometer for
detecting chlorophyll a; L, light sensor; 0PC, optical zooplankton counter; Pl, intake for
zooplankton sampling; SB, stabilizer; STD, salinity—temperature—depth sensor;

T, towing arm.

Figure 4.3 A scuba diver counting the number
of zooplankton (larvacea) contained within a
frame of known dimensions.

Further, it is now recognized that some zooplankton are capable of avoiding
towed samplers, which they detect either visually or by sensing turbulence
created in advanee of the moving gear. In addition, certain gelatinous
plankton are so fragüe that they are impossible to collect intact in nets;
others disintegrate rapidly in the preservatives that are routinely used to store
collections. Crustaceans usually comprise the majority of zooplankton in net
collections because most are too small to avoid capture, and because their
hard exoskeletons protect them from damage and distortion in nets and
preservatives. Thus the numbers of Crustácea relative to other types of
zooplankton may be overestimated, and net-collected plankton may not
provide a true representation of the natural plankton community in many
areas. The numerical dominance and biomass contribution of crustaceans,
especially copepods, needs to be reassessed in many localities.

Zooplankton can also be observed directly in the field, either by scuba
diving down to depths of about 30 m (Figure 4.3) or, in deeper waters, by
using manned submersibles or ROVs (remotely operated vehicles) which are
tethered to ships and coupled with underwater video cameras of high
resolution. These techniques have resulted in discoveries of new species,
particularly of fragüe forms; in an awareness of the problem of
underestimating numbers and biomass of these animáis from net collections;
and in new behavioural observations of many species. Bioacoustic methods,
developed from the use of sonar to lócate fish schools, are also being applied
to lócate and estimate densities of larger zooplankton, such as euphausiids,
which form dense aggregations.

4.2 H0L0PLANKT0N: SYSTEMATICS AND BI0L0GY

There are approximately 5000 described species of holoplanktonic
zooplankton (excluding protozoans) representing many different taxonomic
groups of invertebrates (Table 4.1). Those groups that are commonly found
in the sea and that form significant fractions of the plankton community are
described below. In addition to providing descriptive anatomical accounts,
particular attention is given to describing the food and feeding mechanisms
of each group as these are important in the discussions of food webs and
energy transfer which follow.

The smallest of the zooplankton are certain unicellular protists (Table 4.1).
Included are many species of dinoflagellates that are partly or wholly
heterotrophic (see Section 3.1.2 for a discussion of autotrophic species).
These heterotrophic dinoflagellates feed on bacteria, diatoms, other
flagellates, and ciliate protozoans that are either drawn to the predator by
flagella-generated water currents, or that are trapped in cytoplasmic
extensions of the dinoflagellate. Some species are only capable of
functioning as heterotrophs; others that contain chloroplasts may also
function as autotrophs part of the time. The best known heterotrophic
dinoflagellate is Noctiluca scintillans (Figure 4.4), which has the form of a
gelatinous sphere 1 mm or more in diameter. Noctiluca often occurs in
dense swarms near coasts, and it feeds on small zooplankton (including fish
eggs) as well as on diatoms and other phytoplankton.

A taxonomically diverse group of flagellated protists, commonly called
zooflagellates, ineludes all those species that are colourless and strictly
heterotrophic. All of the organisms in this group lack chloroplasts and plant

Table 4.1 Major taxonomic groups and representativas of holoplanktonic
zooplankton.

Phylum

Subgroups

Common genera

Protozoa

Dinoflagellates

Noctiluca

(= heterotrophic

Zooflagellates

Bodo

protists)

Foraminifera

Globigerina

Radiolaria

Aulacantha

Ciliates

Strombidium; Fave lia

Cnidaria

Medusae

Aglantha; Cyanea

(formerly

Siphonophores

Physalia; Nanomia

Coelenterata)

Ctenophora

Tentaculata

Pleurobrachia

Nuda

Be roe

Chaetognatha

Sagitta

Annelida

Polychaetes

Tomopteris

Mollusca

Heteropods

Atlanta

Thecosomes

Limacina; Clio

Gymnosomes

Clione

Arthropoda

(Class Crustácea)

Cladocera

Evadne; Podon

Ostracods

Conchoecia

Copepods

Calañas; Oithona

Mysids

Neomysis

Amphipods

Parathemisto

Euphausiids

Euphausia

Decapods

Sergestes; Lucifer

Chordata

Appendicularia

Oikopleura

Salps

Salpa; Pyrosoma

pigments, and many feed on bacteria and detritus. Although they are very
small (typically 2-5 ¡jl m), they have potentially high reproductive rates and
therefore can become exceedingly abundant under favourable environmental
conditions. Heterotrophic flagellates account for 20-80% of the
nanoplankton by cell number, and thus they may be an important food for
zooplankton that feed on small organisms.

Figure 4.4 Noctiluca, a heterotrophic
dinoílagellate. (Diameter, 1 mm.)

Figure 4.5 Hastigerina pelágica , a pIanktonic

foraminiferan. A bubble capsule of cytoplasm
surrounds a central shell with numerous
radiating spines. Fine strands of cytoplasm,
known as rhizopodia, project along the spines.
The rhizopodia have sticky surfaces and are
used to snare prey such as small copepods and
a variety of microzooplankton. (Diameter is
approximately 3 mm.)

Marine amoebae inelude the Foraminifera (Figure 4.5), which are
characterized by having a calcareous perforated shell, or test, that is usually
composed of a series of chambers. The size range of planktonic species is
about 30 /xm to a few millimetres. Food, consisting of bacteria,
phytoplankton or small zooplankton, is captured by specialized slender
pseudopodia (called rhizopodia) that project through the pores of the test.
Although there are less than 40 known planktonic species (but ca. 4000
benthic species), these holoplanktonic foraminifera are very abundant,
particularly between 40°N and 40°S where they generally inhabit the top
1000 m of the water column. After death, the shells of these protozoans sink
and accumulate in large quantities on the seafloor, forming a sediment
known as foraminiferan ooze.

The Radiolaria (Figures 1.7, 4.6 and Colour Píate 9) are spherical,
amoeboid protozoans with a central, perforated capsule composed of silica.
Most are omnivorous, and they have branched pseudopodia (called
axopodia) for food capture; prey ineludes bacteria, other protists, and tiny
crustaceans as well as phytoplankton (especially diatoms). The size of
individual organisms ranges from about 50 /xm to as much as several
millimetres in diameter; some species form gelatinous colonies composed of
many individuáis and up to a metre or so in length. Radiolaria occur in all
oceanic regions but are especially common in coid waters, and many are
deep-sea species. A sediment composed of the siliceous remains of these
protozoans is called radiolarian ooze.

Planktonic ciliates (Colour Píate 10) are present in all marine regions and
are often extremely abundant. All use cilia for locomotion, and some have
modified oral cilia used for food capture. Ciliates can feed on small phyto-
and zooflagellates, small diatoms, and bacteria. Tintinnids (Colour Píate 11)
make up one large subgroup (>1000 species) of marine ciliates. They are
noted for their vase-like external shells that are composed of protein;
because this substance is biodegradable, the shells are not present in
sediments. Despite their small size (about 20-640 /xm), tintinnids are of
considerable ecological significance as they are widely distributed in both

Figure 4.6 A large radiolarian with a central spherical skeleton composed of silica.
Numerous axopodia radíate from the central capsule of cytoplasm that lies within the
sponge-like skeleton. Prey such as tintinnids, small copepods and other microplankton are
captured by the sticky surfaces of the axopodia. Many of the white spots in the photo are
algae that typically live in association with this radiolarian. (Size is approximately 1 mm.)

open seas and Coastal waters, where they feed primarily on nanoplanktonic
diatoms and photosynthetic flagellates. In Coastal waters, tintinnids may
consume 4-60% of the phytoplankton production. In turn, they are prey for
a wide variety of mesozooplankton.

Jellyfish, or medusae (Colour Píate 12), are conspicuous and common
inhabitants of both the open sea and coastal waters. Some species are
holoplanktonic, but others have an asexual benthic stage in their Ufe cycle and
thus their medusae are part of the meroplankton. Although jellyfish belong to
several different taxonomic groups within the Phylum Cnidaria, all are
characterized by a primitive structural organization, and all are carnivorous,
capturing a variety of zooplanktonic prey by tentacles equipped with stinging
cells called nematocysts. They range in diameter from just a few millimetres
to 2 m for Cyanea capillata , a common northern species with 800 or more
30-60 m-long tentacles. Some well-known pelagic Cnidaria are colonial
forms, like siphonophores (Figure 4.7 and Colour Plates 13 and 22), in
which many individuáis with specialized functions are united to form the
whole organism. Physalia , or the Portuguese man-of-war, is a tropical
siphonophore that floats at the surface with its tentacles extending as far as
10 m below; it is capable of capturing sizeable fish, and its stings can be
painful to swimmers. However, the medusae known as box jellyfish are much
more dangerous. Chironex fleckeri of tropical Australia is the most venomous
animal on Earth, and this ‘sea wasp’ has caused at least 65 human fatalities in
the last century. The stings of a large individual, with up to 60 tentacles
stretching some 5 m, can cause death within four minutes. In nature, Chironex
uses this potent venom to quickly kill prey such as shrimp.

Figure 4.7 Siphonophores: (a) a surface floating species, Physalia physalis, the Portuguese
man-of-war, with tentacles up to 10 m long; (b) a swimming species, Nanomia sp., ca.

10 cm long. Both species use their long trailing tentacles to capture prey.

Ctenophores are closely related to jellyfish, but their structure is sufficiently
different to warrant their being placed in a sepárate phylum. These are
transparent animáis that swim by means of fused cilia arranged in eight rows
(called comb plates). Like the Cnidaria, ctenophores are carnivores, but they
lack the nematocysts of their cióse relatives. Certain of the ctenophores like
Pleurobrachia (Colour Píate 14) have long paired tentacles with adhesive
cells that are used to capture prey; other species (e.g. Bolinopsis) capture
food in large ciliated oral lobes. These ctenophores can have significant
impacts on fish populations as they feed directly on fish eggs and fish larvae,
and they also compete with young ñsh for smaller zooplankton prey such as
copepods. Some ctenophores like Beroe (Colour Píate 15) lack tentacles but
have large mouths; they engulf their prey, which consists principally of
tentaculate ctenophore species.

Chaetognaths, or arrow worms, (Figure 4.8), are one of the best known and
most abundant carnivorous planktonic groups. These hermaphroditic animáis
are found only in the sea, down to depths of several thousand metres. They
have transparent, elongate and streamlined bodies, and most are less than
4 cm long. They often remain motionless in the water, but are capable of
swift darting motions when in pursuit of prey. Food, consisting of smaller
zooplankton, is captured by clusters of chitinous hooks located around the
mouth of the predator. Chaetognaths do not seem to be selective in prey
type; often the type of food eaten reflects local relative abundance of suitable
prey. There are a few other holoplanktonic worms belonging to different
phyla but, in most regions, these are generally found in very low numbers.

Figure 4.8 The chaetognaths (a) Sagitta
pulchra and (b) Sagitta ferox.

Figure 4.9 The planktonic polychaete
Tomopteris helgolandica with múltiple swimming
legs and long, slender, paired antennae. Length,
45 mm.

One exception is the polychaete (Phylum Annelida) genus Tomopteris
(Figure 4.9) with about 40 species (all predators) distributed throughout the
world ocean.

•UEST10N 4.1 Chaetognaths, ctenophores. sonie jellytish» and many oiher
zooplankton living in the upper layéis of the sea are transparent, Can yon
suggest whv iranspareney is a use ful characteristic for animáis living in the
euphotic /one?

Only a few species belonging to the Phylum Mollusca have become
holoplanktonic. Heteropods are a small group ( ca. 30 species) of molluscs
that are closely related to snails, but these pelagic forms swim by undulating
motions of a single fin developed from the creeping foot of their benthic

ancestors. Some of the species can completely withdraw into a small
(<10 mm) external, spirally-coiled shell (Colour Píate 16); others have
reduced shells or lack shells entirely and are highly transparent animáis that
may attain lengths of up to 50 cm (Colour Píate 17). Despite these external
differences, all heteropods have remarkably well-developed eyes and are
visual predators, feeding on other planktonic molluscs, or on copepods,
chaetognaths, salps, or siphonophores. Prey are actively pursued and captured
by large chitinous teeth which can be protruded from the mouth. Heteropods
are generally found in oceanic warm water areas, but, like many carnivorous
plankton, they are not very abundant in any given locality. Their calcium
carbonate shells are sometimes found in sediments on the ocean floor.

The shelled pteropods, or thecosomes, are also holoplanktonic snails. Most
of them have a thin, external, calcareous shell measuring from a few to
about 30 mm in largest dimensión. The shell is spirally coiled in primitive
species but assumes a variety of shapes in more advanced members (Colour
Plates 18, 19). One subgroup (called pseudothecosomes) (Colour Píate 20) is
made up of larger animáis (>30 mm) that lack a true shell; instead, there is
a cartilaginous, internal, skeletal structure. All thecosomes swim by means
of paired wings or a fused wingplate, structures that developed from the foot
of benthic molluscan ancestors. Despite considerable structural diversity, all
thecosomes are suspensión feeders. They produce large, external, mucous
webs that are held in the water while the animal remains motionless below.
As the web filis with organisms entangled in the sticky strands of mucus, it
is withdrawn and ingested. Food consists of phytoplankton as well as small
zooplankton and detrital material. Some of the shelled pteropods can be very
abundant in epipelagic areas, particularly those species that inhabit polar
seas. Some thecosomes are an important food source for pelagic fish,
including commercially important species like mackerel, herring, and salmón.
The shelled pteropods have an unusual reproductive pattern in which an
animal is first a male that mates with another male; the sperm is stored until
the animal changes into a female that lays fertilized eggs in mucoid floating
masses. The carbonate shells of dead animáis eventually sink and accumulate
in certain areas to form a type of sediment known as pteropod ooze.

Thecosomes are preyed upon by another group of planktonic gastropod
molluscs, the naked pteropods or gymnosomes. These animáis lack shells as
adults, but they too swim by means of paired wings. Of the approximately
50 species, Clione limacina (Colour Píate 21) is the largest (to 85 mm long),
most abundant, and best known. It lives in polar and subpolar regions of the
Northern Hemisphere, where it feeds only on several species of the
thecosome genus Limacina (Colour Píate 18). Other gymnosomes also are
predators that feed exclusively on specific shelled pteropods. All capture
prey with special tentacles and chitinous hooks, and they remove the soft
parts of the prey from its shell before swallowing it.

The segmented Crustácea are represented in the sea by several different
groups, but copepods are the predominant forms. Some of the most
abundant and best known marine zooplankton belong to the Order
Calanoida (Figure 4.10a-c) which comprises about 1850 species. These
free-living calanoid copepods are present in all marine regions and usually
make up 70% or more of all net-collected plankton. All species have three
distinctive body regions: the head and first segment of the body are fused
and bear two pairs of antennae and four pairs of mouthparts; the segmented
mid-body has paired swimming legs; and the narrow posterior section lacks

1 mm

Figure 4.10 Planktonic Crustácea. Calanoid
copepods: (a), Pseudocalanus eiongatus: (b),
Centropages typicus ; (c), Calanus finmarchicus.
(d), Microsetelía norvegica, a harpacticoid
copepod. (e), Oithona simi/is, a cyclopoid
copepod. Cladocera: (f), Podon leuckartr, (g),
Evadne nordmanni. (h), Conchoecia eiegans, an
ostracod. Euphausiids: (i), Thysanoessa inermis ;
(j), Meganyctiphanes norvegica. (k), Themisto
abyssorum, an amphipod. (I), Gnathophausia
zoea , a mysid.

appendages. Total body length is usually less than 6 mm, but some
exceptional species exceed 10 mm in length. Many of the species feed by
capturing phytoplankton, especially diatoms, in currents generated by
movements of the swimming legs and mouthparts. Some calanoids, however,
are omnivorous or carnivorous and feed on small zooplankton. The sexes are
sepárate, and fertilized eggs may be laid freely in the water or may be
retained in an external cluster by the female. Development involves twelve
different stages, each separated by moulting, or casting off of the
exoskeleton, and marked by the appearance of new segments and additional
appendages. The first six stages are nauplius (plural, nauplii) larvae
(designated NI to NVI); the last six are copepodite stages (CI to CVI), with
CVI being the sexually mature adult.

Another copepod group, the Order Cyclopoida (Figure 4.10e), differs in that
members have relatively shortened antennae and more segments in the

posterior third of the body. The order contains over 1000 species, but the
majority live among benthic algae or in bottom sediments; only about 250
species are planktonic. Small species belonging to the planktonic genera
Oithona and Oncaea can, however, be very abundant. Some of the
cyclopoid copepods have specially modified antennae for capturing
individual microzooplankton.

The majority of copepods belonging to the Order Harpacticoida
(Figure 4.10d) are Coastal or live in association with the sea bottom.
Approximately 20 species are holoplanktonic, and these are characterized by
usually being very small (<1 mm long) and without distinct divisions
between body regions. Although some species are widely distributed and
may be seasonally or locally abundant, their ecological importance in the
plankton community does not seem to be great.

The euphausiids (Figure 4. lOi-j) form another important group of marine
Crustácea with 86 species. These shrimp-like animáis are generally of
relatively large size, with many species attaining a length between 15 mm
and 20 mm and with some exceeding 100 mm. Euphausia superba is the
krill of the Antarctic Ocean, where this abundant species is a major
component of the diet of many larger animáis and is itself harvested
commercially (see Sections 5.2 and 6.1). Euphausiids are fast-swimming and
are usually undersampled by large nets because of their visual perception and
avoidance capabilities. But it is known that euphausiids form major fractions
of the zooplankton biomass in the open ocean of the North Pacific and North
Atlantic and in the Arctic, and they are important food for fish (e.g. herring,
mackerel, salmón, sardines, and tuna) and some seabirds in these areas.
Euphausiids are generally omnivorous with food consisting of detritus,
phytoplankton, and a variety of smaller zooplankton. Larger species are also
capable of feeding on fish larvae. Euphausiids, like copepods, have a series
of anatomically distinct larval stages separated by moulting and growth.

Amphipods (Figure 4.10k) are distinguished from other Crustácea by having
laterally compressed bodies. They usually constitute only a small fraction of
the total zooplankton. Parathemisto gaudichaudi is a common pelagic
species with a wide distribution at relatively high latitudes in both
hemispheres. The adults of this species are free-living carnivores, feeding on
copepods, chaetognaths, euphausiids, and fish larvae. Many pelagic
amphipods, however, are commonly found attached to siphonophores
(Colour Píate 22), medusae, ctenophores, or salps, and the amphipods either
feed as predators on these animáis or act as parasites. In contrast to
copepods and euphausiids, amphipods have direct development; the young
are released from a brood pouch and look like miniature adults.

Ostracods (Figure 4.1 Oh) are usually minor components of the zooplankton
community. These crustaceans have a unique, hinged, bivalved exoskeleton
into which the animal can withdraw. Most species are rather small although
Gigantocypris , a deep-water inhabitant, reaches more than 20 mm in
diameter. Little work has been done on feeding habits in this group, but
some species are regarded as scavengers.

Although there are over 400 species of freshwater Cladocera (including
Daphnia , the common water flea), there are only about eight marine species
(Figure 4.10f-g) in this primitive group of crustaceans. The marine
cladocerans are primarily of interest in Coastal and brackish water, although
there are species that become seasonally abundant for brief periods in the

open ocean. Because they are capable of producing cloned offspring by
parthenogenesis (i.e. reproduction without males and without fertilization),
Cladocera are able to rapidly increase their numbers when environmental
conditions are favourable.

Mysids (Figure 4.10/) are Usted in Table 4.1 and mentioned here for
completeness, but they seldom are important components of the plankton
community. Many of these shrimp-like animáis spend part of the time on the
seafloor, but rise into the overlying water at night or when forming breeding
swarms. A few oceanic species are residents of near-surface waters, but most
Uve in deeper zones. The most abundant and best known species are
estuarine or inshore residents, and some of these are harvested commercially
in parts of Asia.

The most advanced Crustácea are decapods, a group that encompasses
shrimp, lobsters, and crabs. Most are benthic, but some are holoplanktonic or
nektonic. Pelagic species inelude about 210 species of shrimps (Colour
Píate 23) that typically measure 10-100 mm or more in length and thus
constitute some of the larger zooplankton. They are strong swimmers and
avoid capture by ordinary plankton nets. Most Uve below 150 m depth in
daytime. They are usually omnivores or predators, and copepods,
euphausiids and other planktonic Crustácea are their predominant foods.
Densities of pelagic shrimp in oceanic areas are typically on the order of 1
individual per 200-2000 m 3 of water, but they can be important prey for
various fish, including albacore tuna, and for dolphins and whales.

Two groups of chordates are important members of marine zooplankton
communities. Appendicularians (Colour Píate 24) are closely related to
benthic tunicates or sea squirts (see Section 7.2.1). Because they closely
resemble the larval stages of these bottom-dwelling relatives, they are also
known as larvaceans. The body of appendicularians looks like a tadpole; it
consists of a large rounded trunk containing all the major organs and a
longer, muscular tail. Most species secrete a spherical balloon of mucus,
called a house, in which they reside. The body is generally only a few
millimetres long, whereas houses range from about 5 mm to 40 mm long.
Movements of the animal’s tail create a current of water that enters the
mucoid house through mesh-covered filters which remove larger particles of
suspended material. As water flows through the house, it passes another
feeding filter where nanoplankton and bacteria are collected and transported
to the mouth. Periodically the filters become clogged with particles and the
house must be abandoned, an activity that can be repeated up to a dozen
times per day; new houses can be secreted within a few minutes. Discarded
houses can reach densities of more than 1000 itT 3 , and they contribute to
the formation of marine snow, a term applied to macroscopic aggregates of
amorphous particulate material derived from living organisms. Abandoned
houses represent rich sources of food and surfaces for attachment by other
organisms in the water column and, for these reasons, they are rapidly
colonized by bacteria and protozoans. Larvaceans grow rapidly, and have
short generation times of 1-3 weeks. They are among the most common
members of the zooplankton, being especially abundant in Coastal waters and
over continental shelves where densities may reach 5000 m~ 3 . The 70 or so
species are distributed in all the oceans.

Salps (Colour Plates 25, 26) constitute another class of chordates, but these
animáis are commonly found only in warm surface or near-surface waters.
Each individual salp has a cylindrical, gelatinous body with openings at each

solitary generation

Figure 4.11 The life cycle of salps. one individual oí the Chain

end. Locomotion is achieved by muscular pumping which propels water
through the body. This water current also brings food partióles into contact
with an intemal net of mucus that is continuously secreted by the animal.
Cilia transport food entrapped in mucus to the esophagus where it is
ingested. Food consists primarily of phytoplankton and bacteria, ranging in
size from about 1 /xm to 1 mm. Because salps often form dense swarms and
have high feeding rates, their feeding activities may significantly reduce the
concentrations of small-sized organisms in the surrounding water. Salps have
an unusual life cycle (Figure 4.11) in which sexual reproduction altemates
with asexual budding. Each species of salp has two different forms: the
asexual form is a solitary individual (1-30 cm long) that buds to produce a
chain of up to several hundred individuáis and as long as 15 m; each
aggregate in the released chain is an hermaphrodite and will produce both
sperm and a single egg. Self-fertilization does not usually occur because the
egg and sperm ripen at different times. Following cross-fertilization, the
single embryo grows within the parent and eventually breaks through the
parental body wall to become a young, free-swimming, solitary individual
which once again will continué the cycle with asexual budding. Salps and
the related appendicularians are good examples of r-selected species (see
Section 1.3.1) with extremely rapid growth rates and short life spans; thus
they can quickly respond to favourable environmental conditions by
producing large populations.

QUESTION 4,2 Can you think of any reasons why salps may have developed a
complex üfe cycle that involves two different reproductive pattems?

4.3 MER0PLANKT0N

Some benthic marine invertebrates have no free-swimming larval stage. Their
young hatch as miniature adults from eggs attached to the sea bottom, or
emerge directly from the parent. But approximately 70% of benthic species
release eggs or embryos into the water column, and the resulting larvae
become part of the plankton community. Depending on the species, these
meroplanktonic larvae may spend from a few minutes to several months (or

even years in exceptional cases) in the plankton before they settle onto a
substrate and metamorphose into the adult form. During this time, the larvae
drift in currents and may be dispersed away from the parent population.

Some of the more common types of meroplanktonic larvae of benthic
invertebrates are illustrated in Figure 4.12. Benthic snails and clams produce
a shelled veliger larva that has a distinctive ciliated membrane (called a
velum) that is used for locomotion as well as food collection. Sessile
bamacles have free-swimming nauplius stages, usually six, which are
similar to the nauplii of copepods and other planktonic Crustácea, but with
characteristic pointed projections on the anterior edges of the exoskeleton;
these naupliar stages are succeeded by a cypris which attaches to a substrate
and metamorphoses to the adult. Starfish, sea urchins, sea cucumbers, and

Figure 4.12 Meroplanktonic larvae of benthic
invertebrates. (a) snail veliger; (b) polychaete
trochophore; (c) late larva of a polychaete;

(d) bipinnaria of a starfish; (e) echinopluteus of
a sea urchin; (f) barnacle nauplius; (g) barnacle
cypris; (h) crab zoea; (i) crab megalopa.

exoskeleton

(g)

cilia on

edge of velum .

ciliary

0,25 mm bands

Shell

ciliary band

0.1 mm

exoskeleton

setae

0.25

larval arms
0.05 mm

other benthic echinoderms have various types of meroplanktonic larvae,
some of which are shown in Figure 4.12. Benthic worms belonging to
different phyla also have distinctive larvae: polychaetes, for example,
produce a trochophore larva, with several bands of cilia, that eventually
develops a segmented body and appendages before settlement. Benthic
decapods, like crabs, typically have a succession of different planktonic
larval stages that are separated by moulting. Crabs usually hatch as a spiny
zoea, which eventually changes before settlement to a megalopa that
resembles a miniature adult. These examples are but a few of the more
common types of larvae that appear temporarily in the plankton community.

Benthic invertebrates living in shallower zones often produce planktonic
larval stages, but deep-sea species commonly lack a planktonic stage and
instead have direct development or brood protection of young. This may be
related to a lack of suitable and abundant suspended food for planktonic
larvae in deep water. In températe and cold-water inshore regions,
meroplanktonic larvae of benthic invertebrates typically appear seasonally in
response to warmer temperatures and increased phytoplankton. In tropical
waters, the reproduction of benthic invertebrates may be more or less
continuous but with peaks of reproductive activity tied to other
environmental events, such as rainfall; in such areas, meroplanktonic larvae
may be present throughout the year, but in differing abundances.

Fish eggs and fish larvae (Figure 4.13) also form an important part of the
meroplankton; they are referred to as the ichthyoplankton. Some fish

u_i

t mm

Figure 4.13 Meroplanktonic fish eggs and fish larvae. (a) anchovy egg; (b) mackerel egg;
(c) myctophid (lantern-fish) egg; (d) cod embryo developing in egg; (e) newly hatched cod
larva; (f) newly-hatched pilchard.

species attach their eggs to substrates; salmonids, for example, typically
place eggs in the gravel of streams and herring deposit eggs on seaweeds or
directly on the seafloor. Many marine fish, however, release free-fioating,
planktonic eggs; these inelude sardines, anchovy, tuna, and many other
commercially harvested species. These planktonic eggs are typically
spherical, transparent, and small, usually of the order of 1 -2 mm in
diameter. The eggs contain varying amounts of clear yolk which is the food
of the developing embryos and newly hatched larvae. Some eggs also
contain oil globules, apparently to aid in fíotation.

As with the meroplanktonic larvae of benthic invertebrates, the appearance
of fish eggs in the plankton is dependent on the spawning eyeles of the
adults, and these are often linked with environmental change. Rate of
development within the egg is species-specific but is also closely tied to
ambient sea temperatures, with hatching being delayed in colder waters.
Hatching generally occurs within a few days to a few weeks after the eggs
are spawned. The numbers of eggs produced may be very large. Each female
plaice, for example, produces about 250000 eggs; haddock lay about
500000; and cod lay over one million. If these figures are multiplied by the
number of spawning fish, the annual production of eggs may be enormous.
Numbers of pilchard eggs in the English Channel alone have been estimated
to exceed 4 x 10 14 . Obviously few of the young from these eggs survive to
adulthood; many form an important food source for the holoplankton and
also for adult fish.

In general, for both lisli and benthic i n vertebrales, ihere is an in verse
córrela t ton between egg size and the numbers of eggs produced. Is the re any
selective advantage to produdng large numbers of small eggs or,
alternadvely, to producing small numbers of large eggs?

Two life patterns are operable with regard to egg size. Some species produce
large eggs, but in small numbers because of size and energy restrictions.
Larger eggs contain more yolk and the newly hatched young will be larger
relative to young from smaller eggs. Larger young tend to have higher
survival rates, probably because they are already too large to be eaten by
some pelagic predators, and possibly because they are also more active and
better able to evade potential predators. On the other hand, many species
produce enormous numbers of small eggs which contain little or no nutritive
material for the developing embryos. These young hatch at a small size; they
are vulnerable to many predators, and it is critical that they immediately
begin feeding. Mortality is much higher for these young compared to those
from large eggs, but the high mortality is compensated for by larger numbers.

For the first few days after hatching, fish larvae retain remnants of the yolk
in a sac carried under their body, and they continué to rely on this yolk sac
as a food source until the mouth and gut develop. When the yolk is
exhausted and the larvae begin to feed, they are totally dependent upon
suitable food being plentifully available in the plankton. This dependeney
lasts throughout their planktonic life, for up to several months, until they are
large enough to be classified as nekton and can actively seek feeding areas
independently of current drift. At the same time, the young are vulnerable to
pelagic predators, both larger zooplankton and nekton. Mortality is high
during the planktonic stages; usually only a small fraction of fish survive to
adulthood. For example, it is estimated that early life mortality in cod is as
much as 99.999%.

4.4 VERTICAL DISTRIBUTION

We have previously classified zooplankton according to size, habitat,
taxonomic position, and length of residency in the plankton community.
Zooplankton are also grouped according to their depth position in the
water column.

Those species that live permanently at the sea surface and whose bodies
project partly into the air are called pleuston. They are sometimes
considered to form a special category because they are passively transported
by wind instead of by currents. The neuston ineludes those species that
inhabit the uppermost few to tens of millimetres of the surface water.
Ecologically, it is difficult to sepárate these categories, and here they are
discussed together as organisms living in the uppermost zone of the ocean.
This community is richly developed in tropical waters, and most of the
foliowing examples are typically warm water species.

Examples of pleustonic species inelude the colonial cnidarians Physalia
(Figure 4.7a) and Velella (Colour Píate 27) and their relatives, all of which
have gas-filled floats that project above the water surface. The long trailing
tentacles of Physalia enable it to capture zooplankton and small fish well
below the sea surface. Velella has short tentacles and its food (copepods,
larval crustaceans, and eggs of fish and euphausiids) is captured from very
near the surface. Despite their stinging tentacles, sea turtles and several
surface-dwelling molluscs feed on these Cnidaria. Janthina (Colour Píate 28)
is a snail that builds a raft of air bubbles encased in mucus; the animal hangs
suspended, upside-down, from this float at the sea surface. It feeds on both
Physalia and Velella as well as on other neustonic animáis. Glaucus (Colour
Píate 29) is a nudibranch (or sea slug) that floats upside-down at the surface
by ingesting air which it then stores in special sacs of the digestive tract. It
too feeds on both Physalia and Velella , and it ingests the nematocysts along
with other tissues. Glaucus has the remarkable ability to absorb the stinging
cells without causing them to discharge, and the nematocysts are then
conveyed by cilia to special sacs in the tips of extemal papillae on the
nudibranch’s dorsal surface. There, they can be employed as a defence
against predators, and they will discharge if the animal is disturbed. Bathers
who come in contact with Glaucus which have washed ashore report painful
stings lasting several hours. Other permanent members of the neuston
community inelude the only inseets found in the open ocean; they belong to
the insect order Hemiptera and the genus Halobates. These wingless water
striders cannot survive immersion in seawater and thus are restricted to
eating other organisms living at the immediate surface, including floating
cnidarians and neustonic species of copepods.

Smaller organisms are also present in the immediate surface layers of the
sea. Bubbles produced by breaking waves accumulate organic material in a
thin surface scum, and this provides a rich substrate for bacteria which often
concéntrate in films. This very thin surface región of only a few millimetres
in depth may contain from 10 to 1000 times more bacteria than in the water
immediately below, and consequently these organisms may be an important
part of the neuston, forming food for large numbers of protozoans such as
tintinnids. The high light intensities inhibit photosynthesis and because few
phytoplankton are found in this zone, the bacteria and protozoa are probably
important components in the diets of grazing copepods. Many types of
meroplanktonic invertebrate larvae and the eggs and larvae of certain fish

species are also commonly present in this zone. Indeed, the eggs of some
fish (e.g. anchovy, mullet) are extremely buoyant, adhering to the surface
film, and the upper few centimetres of the sea support many different types
of fish larvae. Some of these fish remain at all times near the surface, but
others — as well as some other types of zooplankton — are transient
members of the neuston, usually moving to the surface zone at night to feed.
The accumulation of organisms at the sea-air interface creates an important
feeding zone for oceanic birds (e.g. petrels, skimmers, fulmars), many of
which have bilis adapted for skimming this layer (see Section 6.5 and
Figure 6.2).

What special environmental fe atures are presern at the sea air interface to
which neuston ic species niust adapt?

This región receives very high levels of infrared and ultraviolet light, the
latter being detrimental to many organisms. The high light intensities also
inhibit the production of phytoplankton, and consequently few, if any, of the
zooplankton residing in this zone are strict herbivores. In calm weather, this
región may experience the most extreme 24-hour changes in temperature and
salinity, although mixing usually produces a temperature-salinity regime that
is indistinguishable from that of the upper 2-3 m. Permanent inhabitants of
this zone are also exposed to extreme wave action during storms. And
neustonic animáis, like intertidal species, are exposed to both marine and
aerial predators.

Neustonic animáis live in a región of high light intensity. What types of
defences might they have against predators that hLint by sight?

Some of the neuston, especially crab and fish larvae, are highly transparent
and therefore difficult to discern by sight. But many tropical neustonic
animáis (e.g. Velella, Halobates , Janthina, Glaucus ) are strikingly coloured
with violet or blue pigments. It has been suggested that these colours may
provide protection from the high levels of ultraviolet light, but such
coloration also may be effective as camouflage from predators because it
blends closely with the blue colour of tropical oceanic water. Janthina and
Glaucus also exhibit counter-shading; that is, those body surfaces that are
directed downward in the water are lighter in colour than areas cióse to the
surface, and this too may be a predator defence. Marine predators like fish,
which approach from below, would view the lighter undersides of these
molluscs against a background of lightly coloured sky whereas, to aerial
predators, the darker blue upper surfaces of prey would blend with the dark
blue colour of the water. Cryptic coloration is not the only defence
mechanism of neuston. For example, some neustonic copepods have
developed the ability to jump out of the water in response to predators.

A special surface community has developed in the Sargasso Sea. Sargassum ,
a floating seaweed, forms an extensive habitat for an unique association of
more than 50 species of animáis. The total wet weight of Sargassum in the
community has been estimated at between 4 and 10 million tonnes. Many of
the animáis found living in the seaweed are primarily benthic, and these
inelude hydroids, sea anemones, crabs, shrimp, and other Crustácea. Some of
the endemic species (that is, those restricted to this particular habitat),
including some of the crabs and fish, have developed special camouflage
protection by coming to resemble Sargassum weed both in shape and colour.

In all areas, the región immediately below the sea surface and extending to
200 or 300 m is referred to as the epipelagic zone (Figure 1.1). Many
zooplankton are permanent residents of this zone, others migrate into this
región at night. Only the zooplankton that live in depths shallower than
300 m during the daytime are regarded as truly epipelagic. The epipelagic
zone coincides with the euphotic and disphotic zones (see Section 2.1.2 and
Figure 2.5), and it supports a great diversity and abundance of life. Many
herbivores and omnivores inhabit this región; these inelude smaller
crustaceans (such as copepods), the thecosomatous pteropods, salps,
larvaceans, and meroplanktonic larvae. Many of the species are relatively
small, and many are transparent.

The mesopelagic zone (Figure 1.1) lies between the bottom of the epipelagic
región and a depth of approximately 1000 m, and the animáis that live here
in the daytime are called mesopelagic species. Many mesopelagic
zooplankton tend to be larger than their epipelagic relatives. In this deep
nonturbulent water, even the delicate-bodied, transparent, gelatinous
zooplankton become more diverse and increase in size. For example, in situ
observations have revealed a deep-sea larvacean ( Bathochordaeus ) that
makes a house of 2 m in size, and siphonophores have been seen that can
extend to 40 m in length, making them among the longest animáis known.
Some of the animáis of this región, such as euphausiids, are at least partly
herbivorous, and they move upward to the epipelagic zone at night to feed
on phytoplankton. Many of the residents, however, are camivores or detritus
feeders, feeding on larger particles.

Particulate organic material sinking from above, especially faecal pellets,
tends to accumulate at depths of about 400-800 m beeause of the density
gradient (pyenoeline) that is associated with the permanent thermocline. This
rich food source is decomposed by bacteria, but is also consumed by
zooplankton that tend to congrégate in these regions, and the decomposition
and animal respiration result in high levels of oxygen utilization. These
biological activities contribute to the formation of oxygen mínimum layers,
or zones where oxygen concentrations may fall from the normal range of
4-6 mg l -1 to less than 2 mg l -1 , even approaching anoxic conditions in
some areas. Physical factors also are in volved in the formation of oxygen
minimum layers. Oxygen is replenished at the sea surface through contact
with atmospheric oxygen, and is carried into deep areas through the sinking
of oxygen-laden surface water. The oxygen minimum layer thus marks the
intermediate depth of minimal physical replenishment, as well as a zone of
high respiration. Certain species such as the ‘vampire squid’, Vampyroteuthis
infernalis , seem to have become uniquely adapted to live permanently within
the low-oxygen zones; other species migrate in and out of the oxygen
minimum layers.

Many mesopelagic animáis have developed red or black coloration. For
example, all pelagic shrimp living below 500-700 m by day are uniformly
bright red (Colour Píate 23), whereas those living in shallower depths are
transparent or semi-transparent. Many mesopelagic zooplankton (and fish)
also have larger eyes and increased sensitivity to blue-green wavelengths of
light; these are the deepest penetrating wavelengths of solar radiation and
also the spectrum of most bioluminescent light.

QUESTIQN 4.3 What is ihe adaptive sigmficance of red coloration in deep-sea
zooplankton?

Bioluminescence refers to light produced and emitted by organisms
themselves, and it is known in marine species of bacteria, dinoflagellates,
many invertebrates (both pelagic and benthic), and some fish. No
amphibians, reptiles, birds, or mammals possess this property, and only one
freshwater invertebrate is known to be luminous. Although the phenomenon
occurs in many shallow-living marine species, bioluminescence becomes
increasingly important in the deep sea where it is the only source of light
below about 1000 m. In the midwater disphotic zone, more than 90% of the
resident species of crustaceans, gelatinous zooplankton, fish, and squid
emit light.

The biochemistry of bioluminescence is not completely understood and
apparently varíes within different species. But, in general, biologically
generated light results from the oxidation of organic compounds known as
luciferins in the presence of the enzyme luciferase. The Chemicals in volved
in the reaction are synthesized by living cells. However, midshipman fish
(. Porichthys ) are known to acquire luciferin from their diet, by eating
luminous crustaceans; in regions where these crustaceans do not occur, the
fish are nonluminous even though they possess the enzyme luciferase. In the
Chemical reactions of bioluminescence, the energy, instead of being released
as heat (as occurs in most Chemical reactions), is used for the excitation of a
product molecule, called oxyluciferin. This ‘excited’ compound (indicated by
an asterisk below) then releases the energy as a photon, producing light.

luciferase

luciferin + 0 2 -> oxyluciferin* -> oxyluciferin + light

The reaction shown above can be carried out in special cells called
photocytes, or in complex organs known as photophores.

Bioluminescence may be used for various types of communication in the
sea, but in many species the behavioural or ecological role of the light
signáis remains unknown. In some planktonic species, bioluminescent
displays result when the organisms are disturbed and are therefore thought to
be employed as a predator defence. When disturbed, some medusae,
siphonophores, ctenophores, ostracods, and deep-sea squid shed luminescing
tentacles or produce clouds of luminous material as apparent decoys for
predators while the darkened animal itself swims away. In some pelagic
species, bioluminescence may serve as a type of camouflage by acting as a
counter-illumination system which eliminates an animal’s silhouette against
downward penetrating daylight. Animáis with eyes may use bioluminescent
signáis to communicate with other individuáis of their own species.
Euphausiids, for example, may respond to luminescence in other individuáis
to form densely crowded schools when chased by large predators, or to
aggregate for breeding purposes. Some siphonophores and deep-sea fish
employ bioluminescent lures to attract prey within cióse range, thus
eliminating the need to expend energy in hunting for food. The ability to
produce light obviously has evolved independently in many organisms, and
it clearly serves a variety of roles.

Bioluminescence also occurs in some of the bathypelagic species that
inhabit the dark water layers from 1000 to 3000 or 4000 m, and in some of
the abyssopelagic species living below these depths (Figure 1.1). In these
zones, many of the zooplankton and fish tend to be deep red or black in
colour and many have smaller eyes than the mesopelagic species. Away from
the productive surface waters, there are fewer species and fewer individuáis.

zooplankton biomass (mg nrf 3 )

0.01 0,1 1.0 10 100 1000

Figure 4.14 The biomass of net-collected
zooplankton (excluding Cnidaria and salps) from
the surface to 8000 m depth in the Northwest
Pacific Ocean (45°N) (black line) and in the
tropical Pacific (6°S) (blue line) during July.

The general decrease in zooplankton biomass with depth is shown in
Figure 4.14 for several regions in the Pacific Ocean. On average, the
biomass of plankton collected in nets decreases by 1-1.5 orders of
magnitude from the surface to 1000 m, and decreases by another order of
magnitude between 1000 m and 4000 m. This exponential decrease in
biomass with depth is correlated generally with longer generation times and
lower fecundity in deeper-dwelling species.

One other category of vertical distribution has been established to encompass
those pelagic species that live either cióse to the seafloor, or that are
temporarily in direct contact with the sea bottom. These animáis are referred
to either as epibenthic or demersal. This category ineludes many Crustácea,
especially shrimp and mysids, and it also applies to bottom-dwelling fish,
like solé and plaice. Where the water is not very deep, these species too may
move away from the seafloor at night.

The depth classification System given here is based on recognition that
environmental conditions change with increasing depth in the sea, and that
the animáis living at different depths have generally evolved different life
strategies. However, it should be stressed that the vertical ecological zones
described here are arbitrary regions, and that individuáis and species are not
distributed uniformly within these depths (see Figure 4.22). Many animáis
move between the different zones, and the deeper meso-, bathy-, and
abyssopelagic zones are not clearly distinguishable from each other. Further,
the depth distribution of some species may change with latitude. This is
particularly true of those cold-water species that are tolerant of a great range
of hydrostatic pressure. The chaetognath Eukrohnia hamata , for example,
lives near the surface in polar areas, but is found only in deep, coid water in
low latitudes.

4.5 DIEL VERTICAL MIGRATI0N

bathy pelagic

Figure 4.15 A schematic illustration of the diel
migration patterns of pelagic shrimps living in
different vertical zones. Dotted and hatched
areas indícate the depth ranges of the main day
and night concentrations, respectively, of the
different groups. Each group (1-7) is a
composite of different species that occupy
similar depth ranges.

One of the most characteristic behavioural features of plankton is a vertical
migration that occurs with a 24-hour periodicity. This has often been
referred to as diurnal vertical migration. However, diurnal refers to evenís
that occur during daytime; it is the opposite of nocturnal. Diel refers to
events that occur with a 24-hour rhythm. Diel vertical migration (or DVM)
is usually marked by the upward migration of organisms towards the surface
at night, and a downward movement to deeper waters in the daytime. This
phenomenon has been known since the time of the Challenger Expedition
(Section 1.4), but even now we do not have entirely satisfactory explanations
for the widespread occurrence and ecological significance of this 24-hour
rhythmical movement. Diel vertical migration occurs in at least some species
of all the major groups of zooplankton (freshwater species as well), and it is
known in dinoflagellates and in many nektonic species, including both
cephalopods and fish. Diel vertical migration occurs in many (but not all)
epipelagic and mesopelagic species and, although few studies have been
done on deeper-living plankton, it is known in some bathypelagic shrimp
(Figure 4.15).

Because of diel vertical migrations, a comparison of day and night plankton
tows taken in the same area at the same depths will always show differences
in species composition and total biomass. This can be seen, for example, in

number per 1000 m 3 of water

Figure 4.16 Day (solid lines) and night
(dashed lines) distributions of the juveniles and
adults of Euphausia hemigibba at one station in
the California Current.

Figure 4.16 which compares the day and night-time distributions of juvenile
and adult euphausiids off the coast of California.

üllESTION 4.4 What is the general panero of vertical migration displayed
by euphausiids in Figure 4.15. and hovv do the migrations of adults and
juveniles difler?

Each species has its own prefeired day and night depth range, and this may
vary with the life stage (as illustrated in Figure 4.16) or with sex of an
individual (e.g. adult female Calanus finmarchicus are strong migrators,
whereas males are not). The preferred depth range may also change with
season, geographic location, and general weather conditions (e.g. cloudiness,
storm turbulence, etc.). In general, however, there are three patterns shown
by migrating marine zooplankton:

1 Nocturnal migration is characterized by a single daily ascent, usually
beginning near sunset, and a single descent from the upper layers which
occurs near sunrise. This is the most common pattern displayed by marine
zooplankton.

2 Twilight migration is marked by two ascents and two descents every
24 hours. There is a sunset rise to a mínimum night-time depth, but during
the night there is a descent called the midnight sink. At sunrise, the animáis
again rise toward the surface, then later descend to the daytime depth.

3 Reverse migration is the least common pattern. It is characterized by a
surface rise during the day and a night-time descent to a máximum depth.

The vertical distance travelled over 24 hours varíes, generally being greater
among larger species and better swimmers. But even small copepods and
small thecosomes may migrate several hundred metres twice in a 24-hour
period, and stronger swimmers like euphausiids and pelagic shrimp may
travel 800 m or more. Upward swimming speeds of copepods and the larvae
of barnacles and crabs have been measured at 10-170 m h _1 , and
euphausiids swim at rates of 100-200 m h _1 . Although the depth range of
migration may be inhibíted by the presence of a thermocline or pyenoeline,
this is not necessarily so, and an animal may traverse strong temperature and
density gradients, as well as considerable pressure changes, during its
migration.

Diel vertical migrations are responsible for the production of moving deep
scattering layers (or DSLs). These are sound-reflecting layers picked up by
sonar traces. They look like false sea bottoms on echograms (Figure 4.17),
and they were initially believed to be the result of physical phenomena.
However, these layers may move over 24 hours, and their rhythm provided a
clue that they were caused by the movements of animáis. During the day, as
many as ñve scattering layers may be recorded at depths of about
100-750 m. At night, the layers rise almost to the surface and diffuse, or
they merge into a broad band extending down to about 150 m. These deep
scattering layers seem to be most frequently caused by the movements of
larger crustaceans (e.g. euphausiids, shrimp) and small fish that possess
sound-reflecting air bladders (e.g. myctophids), but other zooplankton such
as heteropods and large copepods can occasionally form sound-reflecting
layers too.

i-1 i i-1-1-1-1-1-1- 1 -1-p-1-1-1—“i-1-1-1-r

surface

üsiKi ; r

Figure 4.17 An echogram showing day-time
deep scattering layers produced by euphausiids
(ca. 90-150 m), fish (ca. 75-100 m) and
unidentified animáis {ca. 175 m) in Saanich
Inlet, British Columbia, Cañada. Note that the
fish show up as discrete dots, whereas the
smaller but more abundant euphausiids produce
a more even shading pattern. Note also the
irregularity of the seafloor, with an abrupt rise
from about 225 m to 100 m in the centre of
this transect.

The fact that diel vertical migrations are tuned to the natural light.dark cycle
suggests that changes in ambient light intensity may be of primary
importance as stimuli in initiating and timing the migrations. Light intensity
changes can also act as orienting cues for vertically migrating animáis, as
can gravity and changes in hydrostatic pressure. Natural changes in light
intensity which occur seasonally or even daily (e.g. sunny vs. cloudy days;
dark nights vs. moonlit nights) can alter the depth ranges inhabited by
particular species. Under continuous light in the Arctic summer, migrations
may be totally suppressed. Solar eclipses will cause animáis to begin an
upward migration during the day as light intensity decreases. In the
laboratory, the timing of migrations may or may not change to conform with
experimental alternations of light and dark periods. Factors other than light
may also play a role in initiating the diel migrations; among those suggested
as a causal mechanism is hunger, driving animáis upward toward the more
productive areas under the protective cover of darkness.

While light or other factors may trigger the diel vertical migrations of
pelagic organisms, it does not expía in why so many species should show
this behaviour. Whal is the adaptíve valué of vertical migration?

Many hypotheses have been advanced to answer this question, but it may
not be realistic to insist on a universal mechanism governing diel vertical
migration in all species. It is important to recognize that the hypotheses
discussed below may not be mutually exclusive, and that each may be more
applicable to some species than others.

1 One hypothesis is that animáis which remain in darkness or
near-darkness over 24 hours are less vulnerable to visual predators, and this

can be achieved by a daylight descent. The upward migration returns the
animáis to the surface where food is most abundant.

Considerable evidence supports this hypothesis as being the ultimate cause
of DVM in both marine and freshwater species. It has been shown, for
example, that diel migrations in several zooplankton species may become
more pronounced when predatory fish are more abundant. As well, a
45-year-long study of Metridia lucens in the North Atlantic has shown that
the length of time this copepod was present near the surface varied
seasonally, being shorter in the summer when nights are shorter. However,
when food was most abundant during the spring, the animáis remained
longer at the surface than was predicted from length of daylight; the
importance of obtaining food when it is most abundant seems to override the
importance of predator avoidance at this time.

2 A second idea is that zooplankton conserve energy by spending
non-feeding time in deeper, colder water where metabolic energy demands
are less. It has not been proved that the energy saved in a colder
environment would offset the amount of energy used in swimming during
migration. However, energy required for swimming is very low, generally
only a few percent of basic metabolic energy.

3 A third hypothesis recognizes that zooplankton moving vertically in the
water column are subjected to currents moving in different directions at
different speeds. Thus they encounter a new feeding area each time they
ascend. The new feeding area may contain more or less food than the area
occupied the night before but, by migrating vertically, small organisms of
limited mobility can avoid remaining in an area of little food as well as
overgrazing any very productive area. However, experimental manipulations
of food concentrations produce conflicting results depending on the species.
In some cases, low food levels suppress vertical migration; in other
examples, the reverse is true.

Diel vertical migration has several consequences that are biologically and
ecologically importante One is that, since all individuáis of a species do not
migrate at precisely the same time and to the same depths, a population will
eventually lose some individuáis and gain others. This mixture of individuáis
from different populations enhances genetic mixing and is especially
important in species of limited horizontal mobility.

Another important result of vertical migration is that it increases and hastens
the transfer of organic materials produced in the euphotic zone to deeper
areas of the sea. The ladder-like series of migrating organisms (Figure 4.15)
plays an important role in marine food chains. Each migrating animal
removes food from shallower depths during the night; this material is then
actively transported to deeper areas in the daytime. Herbivores remove
phytoplankton from the euphotic zone, then migrate to deeper areas where
they release faecal pellets and other organic debris and where they may be
eaten by deeper-living camivores. The carnivores and scavengers in tum
carry out vertical migrations at greater depths. The active vertical transpon
of organic materials, either in the form of the animáis themselves or in their
faecal pellets and other wastes is significantly faster than the passive sinking
of organic particles.

4.6 SEASONAL VERTICAL MIGRATIONS

In some species, vertical migration patterns change seasonally and may be
associated with breeding cycles and changing depth preferences of different
stages in the life cycle. In the North Pacific Ocean, the dominant copepods
show dramatic changes in their depth patterns. In inshore waters off the
western coast of Cañada, Neocalanus plumchrus adults do not feed, and they
overwinter at about 300-450 m depth where the eggs are laid between
December and April (Figure 4.18). The eggs float toward the surface, and
nauplii (see Section 4.2) hatch and develop at intermedíate depths. Nauplii
are present in near-surface waters from February to April, and the populadon
matures to the copepodite V stage during March to June when primary
productivity is highest. By early June, stage V individuáis contain large
amounts of lipids accumulated from feeding on phytoplankton, and they
begin to migrate to deeper waters where they will subsist on this stored fat
reserve. There they mature to the adult stage VI, mate, and lay eggs during
the winter. In offshore waters, the life cycle changes somewhat, with
spawning in deep water (>250 m) taking place from July to February and
early copepodite stages first appearing in the upper 100 m in October.
Nevertheless the species continúes to show a seasonal migration between
surface waters, where larval development takes place, and deeper waters,
where mating and spawning occur. A similar pattem of vertical migration
associated with different reproductive stages takes place in Neocalanus
cristatus , a large copepod also common to the North Pacific: adults are
present between 500 m and 2000 m, and spawning occurs in deep water;
younger stages move upward and live mostly above 250 m.

Figure 4.19 shows both the seasonal and diel vertical migrations in two
species of North Atlantic herbivorous copepods, Calañas helgolandicus and
C. finmarchicus. During winter in the Celtic Sea, copepodites V and VI of
both species are distributed fairly uniformly from the surface to about 100 m,

Figure 4.18 A schematic diagram of the life cycle of the copepod Neocalanus plumchrus in
Coastal waters off British Columbia, Cañada. The depth distributions of the eggs, larvae
(nauplii l-VI and copepodites l-V) and adults (copepodite VI) are shown over the course of
one year. C, copepodite: N, nauplius.

depth (m) g depth (m)

(a) Catanas helgolandicus

January April July August September

3í1 20 10 0 10 70 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 %

temperatura (*C)

Catanas finmarchicus

Figure 4.19 Seasonal changes in the day-time (white) and night-time (black) vertical
distribution of copepodite stages V and VI of two species of copepods, Calanus
helgofandicus (a) and C. finmarchicus (b) T in the Celtic Sea. Numbers in each plankton haul
are plotted in 5 m depth intervals as percentages of total numbers (n) present in the haul.
Temperature profiles are shown for the day hauls and apply to both species.

and there is little difference between day and night-time distributions. In
spring (April), both species begin to concéntrate in shallower depths, and
they display diel vertical migrations. In July and August, the thermocline
becomes well established and the two species show a clear separation in their
distributions. Calanus helgolandicus continúes to develop in the warmer
surface zone and to display diel migration, but C. finmarchicus moves
deeper into cooler water beneath the thermocline and shows little difference
in day and night depth preferences. By late September, both species reside in
water deeper than 40 m during the day, and C. helgolandicus maintains its
strong vertical movement toward the surface at night.

QUESTI0N 4.5 Of what advantage is the seasonal change in vertical
migratory patterns to Calanus helgolandicus and C. finmarchicus!

The Antarctic krill (Euphausia superba) also undergoes extensive depth
changes during its life cycle. The eggs of the krill are deposited in surface
waters but sink rapidly to depths of 500-2000 m, where they hatch. The
larvae then gradually float and swim to the surface where development is
completed, and juveniles and adults are found at or very near the surface.

100

The total life span is estimated to be at least 2-4 years. During this time, the
vertical migration of the different stages into currents moving in different
directions results in transport away from, and back toward, the Antarctic
continent. Several common Antarctic species of copepods and chaetognaths
also undergo similar seasonal vertical migrations: in the Southern summer,
the species are present in surface water flowing northward from the area
known as the Antarctic Divergence; in the winter, they are found in a
southerly-flowing deeper current which rises to the surface at the Divergence.

Extensive seasonal migrations are generally undertaken only by species
living in températe and coid waters, or in upwelling regions. The migrations
usually result in young animáis being within the productive surface waters at
a time when they can obtain sufficient quantities of food for growth. In
températe waters, as production declines in the surface waters during summer
and fall, late larval stages or sexually mature adults move to deeper waters.
Here, in colder and unproductive waters, they may enter a State called
diapause in which their metabolism slows and they do not feed. Instead,
they subsist on energy reserves built up during their stay in the surface zone.

Both diel and seasonal vertical migrations place migrants in currents that are
moving in different directions and at variable speeds. Despite this,
populations of marine zooplankton do persist in their own characteristic
geographical regions. A distinctive pattem of seasonal vertical migrations in
a species may ensure retention within an appropriate habitat (as it does with
Antarctic zooplankton), or within a productive upwelling area (as is thought
to be the case for several species associated with annual upwelling cycles off
the western coasts of North America and Africa). Diel migrations may
similarly retain animáis in favourable habitats. For example, the diel
migrations of estuarine species may be attuned to the inward and outward
tidal flows of water to retain the animáis within the estuary. In all such
cases, natural selection favours those individuáis that migrate with
appropriate timing; any individuáis with behavioural pattems that do not
conform to the physical System will tend to be lost from the región. The
corollary to this is that at least some species are known to exhibit different
vertical migratory pattems in different locations, further suggesting that these
pattems can be modified to ensure persistence of populations in particular
geographic regions. What appears to be a confusing variability in migratory
pattems among and within species may be the result of evolutionary
adaptations leading to maintenance of populations in favourable
environments of different current regimes.

4.7 ZOOGEOGRAPHY OF THE HOLOPLANKTON

Zoogeographic studies describe the distributions of living organisms and
investígate the physiological and ecological causes underlying the patterns.
Such studies may also be historical, as the present-day distributions of
marine organisms also have been determined by events and changes taking
place over geological time.

Compared with terrestrial environments, the pelagic realm has few physical
barriers to impede mixing and gene flow between populations of
holoplanktonic organisms. But there are hydrographic barriers between
different water masses which have distinctive complements of
physico-chemical conditions and ecological properties. Some zooplankton

101

have wide distributions encompassing a broad spectrum of environmental
determinants, but others are restricted to such narrow limits of temperatura,
salinity, and other factors that they can be used as biological indicators of
the particular water-mass types they inhabit. The concept of using indicator
species to characterize specific water masses has most frequently been
applied to certain species of foraminifera, copepods, and chaetognaths, as
they are usually sufficiently abundant to be sampled routinely (see, for
example, Figure 6.9).

The sharp north-south temperature gradients set major environmental
provinces in the ocean surface waters as described in Section 2.2.1. Even so,
roughly 50% of all epipelagic zooplankton species extend from tropical and
subtropical waters into at least part of the températe zone. Only about
one-third of the epipelagic holoplankton are restricted to the warm waters of
the tropics and subtropics. Fewer species are restricted to coid and/or
températe waters, but included in this category are several species with
bipolar distribution. Bipolar species are found in both arctic/subarctic water
and antarctic/subantarctic regions, but are not present in intervening regions.
They inelude the pteropods Limacina helicina and L. retroversa , the
amphipod Parathemisto gaudichaudi , and a siphonophore ( Dimophyes
árctica) as well as several diatoms. There are also Arctic and Antarctic
species which are so closely similar that they certainly indicate a common
ancestry. An example of these ‘species pairs’ would be the gymnosomes
Clione limacina (present in the Northern Hemisphere) and C. antárctica
(Southern Hemisphere); although morphologically distinguishable, these
species occupy the same position in polar food chains where both feed
exclusively on Limacina helicina and L retroversa. Bipolarity may have
arisen from animáis being transported in the deep, coid water link between
the north and south polar regions. An alternative theory proposes that
formerly cosmopolitan species were displaced from lower latitudes by
competition with other zooplankton, leaving relict populations in the high
latitudes.

There are fewer latitudinal hydrographic barriers in deeper water (see
Section 2.4), and meso* and bathypelagic plankton generally have relatively
wide distributions. However, they are by no means cosmopolitan; that is,
many species are restricted to one of the major oceans. For example, it is
believed that approximately half of the bathypelagic fauna of the North
Pacific is endemic to that area. About 20% of the deep-living North Pacific
copepods are Antarctic forms, which is not unexpected because the Antarctic
is the source of much of the deep water in all the oceans (see Section 2.4
and Figure 2.17).

The number of epipelagic species decreases from low to high latitudes, a
phenomenon seen also in terrestrial animáis. Most of the different
zooplankton groups have fewer species in coid waters, and some (e.g.
heteropods) have no representatives outside of subtropical boundaries (about
45°N-45°S). As species diversity declines in high latitudes, there is a reverse
tendeney for cold-water epipelagic species to have greater numbers of
individuáis. Although many hypotheses have been proposed to explain these
latitudinal differences in diversity and abundance, the reasons
remain debated.

There are a relatively large number of pelagic marine species that are
described as ‘circumglobal tropical-subtropicaP. These species are present in
warm waters of the Atlantic, Pacific and Indian Oceans. They inelude many

102

of the neustonic animáis such as Janthina species and Glaucus atlanticus
(see Section 4.4), as well as representatives of epipelagic euphausiids,
chaetognaths, amphipods, and other major groups. This wide present-day
distribution can be attributed to the long continuity of the warm water
masses of the world via the ancient Tethys Ocean that existed for several
hundred million years, from the Paleozoic until the Late Tertiary (see
Appendix 1 for dates). Many warm-water species of zooplankton, however,
are restricted to one or two of the main oceans, and their distributions may
be related to present-day barriers (e.g. the Isthmus of Panama) that now
cióse former routes between major oceans.

Do humaos influence the distributions of zooplankton?

Constructions such as the Suez Canal have affected zooplankton
distributions. Since this canal was opened in 1869, about 140 species have
entered the Mediterranean from the Red Sea. Zooplankton distributions also
may be changed by accidental transpon in the ballast water of ships (see
also Section 9.3). This is believed to be the way in which the ctenophore
Mnemiopsis leidyi moved from the Atlantic coast of North America into the
highly polluted Black Sea. Within two to three years, this predatory species
increased to a biomass of about 10 9 tonnes in 1990. Its effect on biological
communities and fish stocks in the Black Sea is estimated to be greater than
that of all other anthropogenic factors.

The North Atlantic and North Pacific Oceans have been separated by the
land mass of North America for approximately 150-200 million years. Yet
the températe zones of these oceans show many similarities in their resident
fauna (Table 4.2). The medusa Aglantha digitale , the chaetognath Eukrohnia

Table 4.2 Numerically dominant net-collected zooplankton species in the epipelagic
zones of the northem North Atlantic and northem North Pacific. Species aligned
between the two columns are found in both oceans.

Group

North Atlantic Ocean

North Pacific Ocean

Cnidaria

Aglantha digitale

Annelida

Tomopteris septentrionalis

Chaetognatha

Eukrohnia hamata

Sagitta serratodentata

Sagitta maxima

Sagitta elegans

Amphipoda

Parathemisto pacifica

Copepoda

Calanus finmarchicus

Calanus pacificas

Calanus helgolandicus

Neocalanus plumchrus

Euchaeta norvegica

Neocalanus cristatus

Pleuromamma robusta

Eucalanus bungii

Acartia clausi

Acartia longiremis

Metridia lucens

Metridia pacifica

Oithona spp.

Oithona similis

Oncaea spp.

Scolecithricella minor

Heterorhabdus norvegicus

Pseudocalanus minutus
Paracalanus parvus

Euphausiacea

Meganyctiphanes norvegica

Euphausia pacifica

Thysanoessa longicaudata

Thysanoessa longipes

Mollusca

Limacina helicina

Limacina re trove rsa

Clione limacina

Salps

Salpa fusiformis

103

hamata , the pteropods Limacina helicina and Clione limacina are all
examples of species found in both areas, as well as in the Arctic Ocean. On
the other hand, the dominant copepods and euphausiids of the North Pacific
and North Atlantic are mostly of different species, although some are in the
same genera (Table 4.2). Speciation and present-day distributions in these
areas have been influenced by geological changes marked by intermittent
changes in water flow between the Pacific, Arctic, and Atlantic oceans. At
the present time, there is almost no southward flow of water through the
Bering Strait, but there is a flow from the Pacific into the Arctic as there
presumably has been whenever the Strait has been open. There is a much
greater water exchange between the Arctic and the Atlantic. Present-day
distributions suggest that the Arctic was the passageway between the
Atlantic and Pacific for dispersal of many planktonic species. However,
cooling of the Arctic during the Pliocene and Pleistocene periods (see
Appendix 1) may have resulted in the many examples of species with
discontinuous ranges (found in the northern Atlantic and Pacific but not in
the Arctic) and in subsequent evolution into distinct species.

An unbroken circumglobal ocean lies in the Southern Hemisphere between
Antárctica and the continents of Africa, Australia, and South America.
Zooplankton in this broad oceanic area exhibit continuous, concentric
patterns of distribution around the Antarctic continent which conform to
concentric isotherms and the general clockwise circulation pattern. However,
there are also currents that flow northward from the southward toward
Antárctica and, as pointed out in Section 4.6, seasonal vertical migrations in
these currents also serve to maintain populations around the continent. The
pelagic fauna of the Antarctic Ocean is much richer in numbers of species
than that of the Arctic Ocean, and this difference may be related to the
higher productivity of the Southern ocean.

4.7.1 PATCHINESS

Within the geographical boundaries inhabited by any species, the individuáis
of that species are not distributed uniformly or randomly, but are usually
aggregated into ‘patches’ of variable size. This patchiness is true of both
phytoplankton and zooplankton, as well as of other types of marine and
terrestrial species. Patchiness in phytoplankton distribution was introduced in
Section 3.5, where it was related to physical processes that control nutrient
availability and thus plant production on scales ranging from oceanic gyres
to Langmuir cells of circulation. Zooplankton patchiness may be correlated
with phytoplankton concentrations, or it may be caused by other factors.

Small-scale heterogeneity in the horizontal distribution of zooplankton (also
known as microdistribution) is more difficult to detect and study than broad
geographic patterns because of the way zooplankton are collected. Nets are
generally towed through the water for distances ranging from tens of metres
to kilometres, so that the numbers of collected individuáis are averaged over
distances that will mask any smaller-scale patterns. Specially designed
sampling programs have demonstrated microdistributional patterns in
zooplankton, as have direct observations by scuba divers and observers in
submersibles. Patchy distribution on these smaller scales can be explained in
a number of ways, and may be related to physical, Chemical, or
biological events,

Various types of horizontal and turbulent mixing can result in aggregation
or dispersión of planktonic populations. As discussed in Section 3.5, some

104

types of mixing (upwelling) result in elevated surface nutrient concentrations,
high primary production, and increased numbers of zooplankton; other forms
of mixing (downwelling) have the opposite effect on production and
aggregation of organisms. Zones of vertical mixing range in area from very
large shelf-break fronts (Section 3.5.4), to smaller scale coid- or warm-core
rings (Section 3.5.1), to much smaller Langmuir circulation pattems
(Section 3.5.6), all of which can affect zooplankton distribution and
abundance on corresponding spatial scales. Differences in scale are shown in
Figure 4.20, which illustrates changes in numbers of zooplankton (and
phytoplankton) on a scale of kilometres, and in Figure 4.21, which shows
smaller-scale patchiness of zooplankton on a scale of metres. These figures
¿Ilústrate patchiness on a horizontal axis, but zooplankton also form discrete
aggregates in the vertical dimensión. Figure 4.22 shows the vertical
distribution of copepods in the Bering Sea; note the vertical separation of the
species within the epipelagic and mesopelagic zones, as well as the discrete
depths inhabited by different life stages and by males and females.

What are some of the biológica! or ecológica! causes of patchy distribution
of zooplankton?

Patchiness may result from interactions between zooplankton and their food.
On time scales of months, high primary production may result in high
secondary production (as in Coastal upwelling) but, on shorter time scales
and in smaller areas, dense aggregations of phytoplankton and of
herbivorous zooplankton tend to be mutually exclusive (Figure 4.20). This
may result from heavy grazing by the herbivores which reduces the numbers
of phytoplankton. It also may be the result of differences in growth rates
between the algae and the zooplankton; whereas phytoplankton can quickly
multiply under favourable light and nutrient concentrations, increases in
numbers of zooplankton often lag considerably behind because of their
slower generation times. Consequently, when phytoplankton numbers are
peaking and nutrients are declining, zooplankton biomass may still be low as
the animáis begin growing in response to the elevated food supply.

Reproduction may also play a role in causing patchy distribution in some
species. Aggregations of zooplankton formed for purposes of breeding will

Figure 4.20 Patchiness in phytoplankton (as
indicated by chlorophyll a concentraron) and in
zooplankton, on a kilometre scale. Based on
night-time data taken from 3 m depth in the
northern North Sea, May, 1976.

105

distance (m)

Figure 4.21 Examples of small-scale (in metres) patchy distribution of zooplankton off the
California coast. (a) A shelled pteropod. Limacina ; (b) a chaetognaíh, Sagitta ; (c) a copepod,
Corycaeus ; and (d) euphausiid larvae.

cause a small-scale heterogeneous distribution, although the mechanisms in
which the members of the swarm unite are not understood. Also, all the
progeny hatching from one swarm, or even from one egg mass, tend to
remain together for some period of time before they become dispersed.

Considerable attention has been given to the patchy distribution of Antarctic
krill because of their abundance and consequent importance for higher
trophic levels, and because of the potential for commercial harvesting of this
species (see Section 6.1). When feeding, Euphausia superba forms swarms in
which the individuáis are closely packed but move independently of each

depth (m)

106

relativa frequency {%)

Eucaianus bungü +

E . bungü CV
/Wefr/d/a pacifica CV

M. pacifica ?

A/e o c a/an us cr/sfaíus CV
A/, p/umc/íros CV

N. plumchrus O
N. píumchrus ?
Pseudocalanus minutus ¥
P. minutus copepodites
Scoiecithriceiia minar ¥

Figure 4.22 Vertical zonation ot the copepod community in the Bering Sea during summer.
Samples were collected during daylight hours. ?, females;</, males; CV, copepodite stage V
(see Section 4.2).

other. At other times, the euphausiids are organized in schools, in which the
individuáis are uniformly oriented and swim together at a uniform speed.

The formation of schools is thought to offer some defence against certain
predators but, on the other hand, some predators may forcé schooling in
order to concéntrate their prey. For example, some températe-water species
of euphausiids may be driven into tight schools by sharks, or by whales
which produce a ‘net’ of bubbles to encircle and concéntrate their prey. In
general, extremely high concentrations of predators (e.g. swarms of medusae
or ctenophores, or fish schools) will quickly cause local decreases in the
numbers of their prey and thus create a patchy distribution of the prey.

Vertical patches of individuáis and species, as shown in Figure 4.22, will
change over 24 hours as some animáis migrate vertically (see Section 4.5).
In general, migrators tend to be more dispersed during the night, and to form

107

denser aggregations during the day in deeper water. Vertical separation may
be due to physical factors that inelude the presence of pyenoelines and
thermoclines, to light intensity preferences, or to other microenvironmental
differences. Vertical aggregations also may result from the distribution of
preferred food Ítems, from predation, or from other biological factors.

Table 4.3 summarizes some of the physical and biological processes that
cause the patchy distribution of planktonic organisms. As mentioned above,
spatial scales vary from thousands of kilometres to very small-scale patches
of 10 m or less. The length of time a particular patch of plankton may
persist varies according to the cause of the distribution. Very large patches of
zooplankton, such as those caught in rings spun off from the Gulf Stream,
may persist for months or even years. Mating aggregations of
macrozooplankton (e.g. euphausiids) or of nekton (squid, fish) may persist
for only a few days, but the planktonic offspring that hatch from spawning
aggregations may remain together for many days or months because they
will be less independent of water movements than the adults. Patchiness due
to Langmuir circulation will persist only as long as wind velocity and
direction remain constant; and wave action may cause constantly changing
patterns of aggregation and dispersal in near-surface plankton. Thus there is a
continuum in size of patches from very large to very small horizontal scales,
and in persistence of patches from thousands of days to momentary periods.

Table 4.3 Approximate spatial and temporal scales of some important processes
that cause patchy distribution of zooplankton.

Spatial
length scale
(km)

Physical

processes

Biological

processes

Persistence
time scale
(days)

1000+

Gyres (e.g. Sargasso
Sea); continental
upwelling (e.g. Perú
Current); water
mass boundaries
(e.g. Antarctic
Convergence)

Regional ecosystems
defined by the water
mass

1000+

100

Warm and coid
core rings; tidal
fronts; seasonal

Coastal upwelling

Seasonal growth (e.g.
spring blooms); differ-
ential growth between
phyto-and zooplankton

Lunar eyeles (e.g. fish
spawning)

100

Turbulence (e.g.
estuarine mixing;
island wake effeets)

Reproductive eyeles

Grazing/predation

Tidal mixing

Diel events (e.g. vertical
migration)

0.1

Wind-induced
vertical mixing

Physiological adapta-
tion (e.g. buoyaney;
light adaptation)

0.1

0.01

Langmuir circulation;
wave action

Behavioural adaptation
(feeding swarms)

0.01

108

QUESTION 4.6 Referring co Figure 4JO* (a) can you provide any explana!ion
íbr why the amount of ehloropfoyll a general ly inversely correlated with
zooplankton numbers at any locality? ib) Would you expect the numbers of
zooplanklon at these localities and depth to increase or decrease during
daylight hours?

4.8 LONG-TERM CHANGES IN ZOOPLANKTON
COMMUNITY STRUCTURE

Records showing long-term changes in plankton community structure are
available for only a few marine areas, but they indícate that there can be
considerable variation in the abundance and species composition of
zooplankton on decadal time scales. Often these changes in plankton
communities are correlated with changes in atomospheric and marine
climate. Long-term climate changes that significantly alter marine
ecosystems and biological production are known as regime shifts, and
several examples are given below.

Some of the longest zooplankton records come from the north-east Atlantic
Ocean, where continuous plankton recorders (CPRs) have been towed by
commercial ships on regular routes for almost 50 years. Figure 4.23 shows a
general decline in both phytoplankton and zooplankton abundance in this
región over the past 40 years, except for small increases during the early
1980s. A similar pattem has been shown off Southern California, where the
macrozooplankton biomass was 70% lower in 1987-93 than in 1951-57. In
this Coastal area, ocean climate changes were correlated with the decrease in
plankton. During the 40 years of declining plankton biomass, the sea surface

Figure 4.23 Long-term fluctuations in the abundance of phytoplankton and zooplankton in
the north-east Atlantic Ocean. The blue solid line represents standard deviation units from
the long-term annual mean; the blue broken line has been statistically smoothed to show the
average trend.

109

temperature off California increased by about 1.5 C°, and the temperature
difference across the thermocline increased. The increased stratification of
the water column lessened wind-driven upwelling, and consequently the
lower-nutrient regime depressed phytoplankton production and supported
fewer zooplankton. It is not clear whether these changes are part of natural
climatic cycles that will reverse in coming years, or whether the zooplankton
decline is due to global warming. If temperatures continué to increase
globally and stratification increases throughout the oceans, the biological
impacts could be drastic in terms of lowering marine production in areas
where it is presently enhanced by upwelling of nutrients.

Can changes in ocean di mate increase biological production?

At about the same time that plankton biomass was declining in the
California Current, plankton abundance was increasing in other parts of the
Pacific. In the central North Pacific Ocean (ca. 26°-3I o N to 150°-158° W),
total chlorophyll a in the water column doubled from 3.3 mg m~ 2 during
1968-1973 to 6.5 mg m -2 in 1980-85. Farther north, in the subarctic
Pacific Ocean, there was a doubling of zooplankton biomass and a similar
increase in pelagic fish and squid abundance between the periods 1956-1962
and 1980-89. In both of these areas, the enhanced production has been
correlated with an increase in the intensity of winter winds, which increase
vertical mixing and bring more nutrients into the euphotic zone.

There may also be long-term changes in species composition of plankton
communities. In the central North Sea, holoplanktonic calanoid copepods
(see Section 4.2) dominated the mesozooplankton community from 1958 to
the late 1970s. In the 1980s and early 1990s, meroplanktonic larvae of sea
urchins and brittle stars (see Sections 4.3 and 7.2.1) became numerically
dominant and more abundant than any single holoplanktonic species. This
change in zooplankton composition was reflected in a 2- to 8-fold increase in
the abundance of the macrobenthos of the area. The reason for a preferential
increase in benthic echinoderms in the North Sea remains unclear.

4.9 SUMMARY OF CHAPTER 4

1 The marine zooplankton community ineludes many different species of
animáis, ranging in size from microscopic protozoans to animáis of several
metres in dimensión. The holoplanktonic species spend their entire lives in
the pelagic environment; meroplanktonic forms are temporary members of
the plankton, and inelude the eggs and larval stages of many benthic
invertebrates and fish.

2 Although zooplankton are routinely collected by towing fine-meshed nets
through the water, not all species are representatively captured by this
method. Some animáis are too small to be retained in nets, others are
capable of detecting and evading nets, and some species are too fragüe to
survive collection by nets and subsequent processing in Chemical
preservad ves. Direct observations of zooplankton using scuba techniques,
ROVs, or submersibles have greatly increased our knowledge of fragüe
and/or fast-swimming species.

3 The presence of meroplanktonic larvae in the water is linked to the
reproductive pattems of the adults. In tropical regions, meroplankton are
present throughout the year. In higher latitudes, the larvae of benthic

110

invertebrates and fish appear seasonally because reproduction in the adults is
linked to higher temperatures and elevated phytoplankton production.

4 The vertical gradients of temperature, light, primary production, pressure,
and salinity create distinctive environments at different depths in the water
column. These vertical zones (epi-, meso-, bathy-, and abyssopelagic) are
somewhat arbitrary in nature, but different species of zooplankton generally
inhabit discrete depth zones within the ocean. The life styles, morphology,
and behaviour of organisms living deeper in the water column differ from
those exhibited by epipelagic species, and the biomass of zooplankton
decreases exponentially with depth.

5 As light from the Sun diminishes with depth, bioluminescent light
produced by organisms becomes increasingly important as a means of
communication. Many different species display the ability to produce light,
and the biological significance of bioluminescence varíes with the species.
Some use light displays to attract potential prey, others to deter predators;
some may use bioluminescence to attract mates, or to form
reproductive swarms.

6 Although most zooplankton have preferred depth ranges, many species
move vertically in the water column with a diel periodicity. The most usual
pattem is a nocturnal migration in which animáis make a single ascent
toward the surface at night, followed by a single descent to deeper water at
sunrise. The adaptive significance of diel vertical migration may be different
for different species. This behaviour may allow animáis to conserve energy
by remaining in colder waters except when feeding; it may reduce mortality
from visual predators; or it may permit animáis of limited swimming ability
to sample new feeding areas with each ascent.

7 Diel vertical migration has several important biological and ecological
consequences. It probably enhances genetic exchange by mixing the
members of a given population; this results because vertical migrations are
never precisely synchronized among all the members of a population. Some
individuáis begin migrations sooner or later than others, with the result that
some members will eventually be lost from the original group and new
members will be added. Secondly, diel vertical migrations increase the speed
at which organic materials produced in the euphotic zone are transferred to
deeper areas.

8 In high latitudes, extensive vertical migrations may be undertaken on a
seasonal basis, and these are generally linked with reproductive cycles and
development of larval stages. In such migrations, the adults are usually
found in deeper waters during the winter when food is scarce; the
developing young are present in surface waters during the spring and
summer when phytoplankton is plentiful.

9 By moving vertically in the water column, zooplankton enter currents
that are moving in different directions and at different speeds. Thus diel or
seasonal vertical migrations that are attuned to particular current regimes can
result in the retention of populations within favourable localities.

10 Present-day distributions of zooplankton have been established over
geological time and reflect past dispersal patterns as well as the
physiological and ecological requirements of the species.

11 Epipelagic zooplankton are often associated with specific water mass
types, which are established by latitudinal gradients in temperature, salinity,

111

and other physico-chemical factors. Mesopelagic and bathypelagic species
tend to have wider geographic distributions, reflecting increasing
homogeneity in environmental conditions with increasing depth.

12 The numbers of species of epipelagic and mesopelagic zooplankton are
higher in low latitudes, but the numbers of individuáis tend to be relatively
low. The reverse situation is found in high latitudes, where there are fewer
species but with higher abundance.

13 Within the bounds of their geographic regions, zooplankton exhibit
patchy distributions on a wide range of space- and time-scales. Patchiness
may result from responses to physical turbulence or mixing, or to Chemical
gradients such as salinity changes. Patchiness may also result from
interactions between prey and predators, or it may reflect other biological
events such as reproduction.

14 Long-term records indicate that plankton abundance and species
composition may change substantially over decadal time scales. Decreasing
plankton biomass may be caused by climate changes that increase water
stratification and depress upwelling; conversely, in other regions, increasing
winds may enhance nutrient concentrations in the euphotic zone and lead to
increased phytoplankton and zooplankton production.

Now try thefollowing questions to consolídate your understanding ofthis
Chapter.

QUESTION 4.7 Why do the planktonic Crustácea tend to have so munv
difieren! growth si ages?

QUESTION 4.8 Why do so many benthic species of animáis produce
meroplankton ic larvae?

QUESTION 4.9 Which planktonic organisms produce skeletal maten ais that
contribute to sedimenta on the seaRoor?

QUESTION 4.10 Oí ihe major zooplankton groups listed in Table 4.2. which
are predominuntly carnivorous and which are predominantly herbivorous?
(Refer to Section 4.2.)

QUESTION 4.11 Whai is the advantage of patchy distribution of plankion
predators that actively seek out their food?

QUESTION 4.12 In Figure 4.22. only the copepodite V stage of Neocakmus
Cristinas is shown, Where and when would you expect to find adults of ibis
species? (Refer lo Section 4.6.)

CHAPTER 5

ENERGY FLOW

MINERAL CYCLING

5.1 FOOD CHAINS AND ENERGY TRANSFER

Food chains are linear arrangements showing the transfer of energy and
organic materials through various trophic levels of marine organisms. Each
trophic level is composed of organisms that obtain their energy in a similar
manner. The pelagic food chain begins with the phytoplankton; these
autotrophic primary producers, which build organic materials from
inorganic elements, form the first trophic level. Herbivorous species of
zooplankton that feed directly on the marine algae (e.g. Protozoa, many
copepods, salps, larvaceans) make up the second trophic level, and they are
referred to as primary consumers. Subsequent trophic levels are formed by
the carnivorous species of zooplankton that feed on herbivorous species
(secondary consumers like chaetognaths), and by the carnivores that feed
on smaller carnivores (tertiary consumers including many jellyfish and
fish). The total number of trophic levels will vary with locality and with the
total number of species in the community. The highest trophic level is
occupied by those adult animáis that have no predators of their own other
than humans; these top level predators may inelude sharks, fish, squid, and
mammals. The total amount of animal biomass produced in all higher trophic
levels, per unit area and per unit time, is called secondary production (as
opposed to the primary production of plants). Trophodynamic studies
examine the factors that affect transfers of energy and materials between
trophic levels and that ultimately control secondary production.

Elements such as nitrogen, carbón and phosphorus, which become
incorporated in organic components of plant and animal tissues, have a
eyelieal flow through food chains (Figure 5.1). Bacteria decompose waste
materials and the tissues of dead organisms. Decomposition releases
inorganic forms of essential elements, and these become available again for
uptake by autotrophic organisms. Energy, however, has a unidirectional flow
(Figure 5.1). Some energy is lost at each transfer to the next trophic level

Figure 5.1 A schematic representation of
mineral reeyeling and energy flow in marine
ecosystems.

heat

113

because much of the Chemical energy incorporated in organic compounds is
converted to heat energy and is dissipated in respiration, when organic
carbón is broken down to CO 2 . As a consequence, the total energy will
diminish at each trophic level, and this places a finite limit on the possible
number of trophic levels in any community.

question 5.1 Whai are the cunsequences of energy loss due to respiration on
(a) the rehit i ve total numbers of organisms in suecessive trophic levels* and
on (b) the relative amounts of primary produciiori and secondary production?

The size of individual organisms generally increases within each succeeding
trophic level, but the generation time (or length of the life cycle) becomes
progressively longer. The generation times of phytoplankton are measured in
hours or days, those of zooplankton in weeks to months; those of fish in
years; and those of marine mammals in many years. One might expect to
observe considerable differences between the standing stocks of
phytoplankton and fish or whales, but it is believed that because of the
dissimilarities in generation times, the total biomass in each succeeding
trophic level decreases only very slightly (Figure 5.2). The biomass of the
exceedingly numerous, microscopic, and rapidly reproducing phytoplankton
is probably never more than four times that of the small numbers of very
large marine mammals which have long generation times.

We have seen that it is relatively easy to estimate primary production by
marine phytoplankton (see Section 3.2.1). Because of the longer generation
times of zooplankton and fish, and because it is difficult or impossible to
follow populations of these animáis in the sea for any length of time, it is
much harder to obtain estimates of pelagic secondary production. There are
some methods that can be applied to data collected in the field, and these are
discussed in Section 5.3.1, along with their limitations. Secondary
production can also be studied by growing marine zooplankton and fish
under experimental conditions, and this approach is considered in
Section 5.3.2. Another approach is to use estimates of primary production

Figure 5.2 The average biomass of organisms of different sizes in marine food chains. To
elimínate differences in shape, the size of each type of organism has been converted to the
diameter of a spherical particle having the same biomass as the organism. The upper line
illustrates the average biomass of different organisms in the Antarctic Ocean, a región of
high productivity. The lower line shows the biomass of typical organisms in the equatorial
Pacific, an oceanic región of lower productivity.

114

and our knowledge of marine food chains to predict secondary production
and yields of fish. Using this indirect approach, it is necessary to know how
much energy is transferred between each trophic level.

The efficiency with which energy is transferred between levels is called the
ecological efficiency (£), and it is defined as the amount of energy extracted
from a given trophic level divided by the energy supplied to that trophic
level Ecological efficiency is difficult to measure, and it can more easily be
approximated from the transfer efficiency (£>)> defined as:

Et = ~ (5.1)

/Vi

where P t is the annual production at trophic level /, and /Vi is the annual
production in the preceding trophic level í — 1. In this equation, production
can be defined either in terms of energy (e.g. measured conventionally in
calories or, in the SI system, in joules) or in terms of biomass (e.g. carbón in
grams). For the energy transfer between phytoplankton and zooplankton, Ej
will equal the amount of herbivore production divided by the primary
production. At the next step, the transfer efficiency will be the annual
production of secondary consumers (i.e. camivorous zooplankton) divided by
the annual production of herbivores.

QUESTION 5,2 Calcúlate the transfer efficiency between phytopEankion and
herbivorous zooplankton in a marine community where the net primary
productivity is 150 g C ni 2 yr~ 1 and the annual production of herbivorous
copepods is 25 g C m" 2 .

In marine ecosystems, valúes for transfer efficiencies have been estimated at
about 20% for the transfer from plants to herbivores, and at 15-10% at
higher levels. This means that there are corresponding energy losses between
trophic levels of 80-90%, primarily through respiration (the heat losses
shown in Figure 5.1).

QUESTION 5.3 Why would the respiral ion losses be § reaten and transfer
efficiencies lower, when considermg higher trophic levels?

Note that equation 5.1 deais with the consumption of energy in succeeding
trophic levels. A certain amount of production is not consumed directly. Not
all living phytoplankton and zooplankton are eaten; some die naturally, and
the energy contained in dead organic material becomes available for
scavengers or microbial decomposers in a different pathway (see
Section 5.2.1). This detritus may be cycled either in the water column or in
the benthic community.

QUESTION 5.4 If a large amount of ihc primary production ¡s not eaten by the
herbivorous zooplankton. hut dies and sinks out of the water column, what
happens to the valué of the transfer efficiency between the se iwo trophic
levels?

In addition to knowing how much energy is transferred between trophic
levels, it is also necessary to know how many trophic levels there are in any
particular locality for which secondary production will be estimated. Because
of the energy losses incurred with each transfer between trophic levels, the
number of links will partly determine the biomass of top-level predators (i.e.

115

fish, squid, or marine mammals). There is reasonable evidence to suggest that
the number of links in the pelagic food chain varíes with locality and may
be determined by the individual size of the primary producers. The number
of trophic levels vanes from up to six in the open ocean, to about four over
continental shelves, to only three in upwelling zones, as shown in Figure 5.3.
Note that when the size of the dominant phytoplankton is small, the food
chain is lengthened, as in open ocean areas. In such situations, marine
protozoans (zooflagellates and ciliates) become important intermediary links;
they may consume a major fraction of the primary production, and in tum
they constitute an abundant dietary source for suspension-feeding copepods
or other zooplankton that are incapable of feeding directly on very small

Figure 5.3 A comparison of food chains in three different marine habitats. The organisms
representing each trophic level are only selected examples of the many marine species that
could be present in that level. (Organisms not to scale.)

I. Open ocean (6 trophic levels)

nanoplankton microzooplanktonmacrozooplankton megazooplankton zoúpíanktivorous fish piscivorous fish
(flagehates) (protozoa) ** (copepods) ^ (chaetognaths) ** (myctophíds) —(tuna, squid)

IL Continental shelves (4 trophic levels)

^ y v

microphytoplankton
(diatoms, dinoflagellates) ;

macrozooplankton
(copepods)

benthicherbivores
'{clams, mussels) -

III. Upwelling regtons (3 trophic levels)

planktivorous fish
(anchovy)

megazooplankton

(krill)

planktivorous whales
(baleen whales)

116

Table 5.1 The relation between recent estimates of primary production and fish
production in three different marine habitats.

Habitat

Oceanic

Coastal

Upwelling

Percent

1.0

ocean area

Mean primary
productivity

300

500

(g C rrr 2 yr" 1 )

Total plant
production
(10 9 tonnes C yr -1 )

1.8

Number of energy
transfers between

1.5*

trophic levels

Average

ecological

efficiency

10%

15%

20%

Mean fish

0.75

1000

44 700

production**

(mg C m" 2 yr -1 )

Total fish

0.24

36.2

162

production***

(10 6 tonnes C yr -1 )

*The number of trophic levels in upwelling areas may be 2 (if fish are predominantly
herbivores) or 3 as represented in Figure 5.3; 1.5 represents an average valué for the
number of energy transfers {n — 1 or 2).

**Calculated from equation 5.2.

***Corrected for percent ocean area occupied by each habitat (total
area — 362 x 10 6 km 2 ).

phytoplankton. In contrast, large chain-forming diatoms domínate in
nutrient-rich upwelling regions, and short food chains result because large
zooplankton or fish can feed directly on the large primary producers.
Consequently, there is a high biomass of top-level predators in upwelling
systems or other highly productive areas. Figure 5.2 compares biomass and
length of food chains in regions of high and low productivity.

QtJESTION 5.5 Refer to Figure 5.2. (a) How many orders of magnitude
difference are there i ti the amounts of biomass produced in the Antarctic
Ocean and in the equatoriaJ Pacific? (b) Whal factors contribute to this
difference? (Refer al so to Figure 5.3 and Seetion 3.5.)

For a given locality, the number of trophic levels can be coupled with a
quantitative estimate of primary productivity to predict yields of secondary
production in any particular trophic level (P( n + 1 >), according to the following
equation:

P {n + X ) = P\E n

(5.2)

In this equation, P\ is annual primary production, E is the ecological
efficiency, and n is the number of trophic transfers (which equals the
number of trophic levels minus 1). A major difficulty in applying this
equation lies in the accuracy of the valúes used for ecological efficiency and
number of trophic levels. For example, the secondary production estimate
can increase by an order of magnitude if the valué for E is doubled. Until
we can be more confident of the valúes for ecological efficiency and the

117

number of trophic levels in different locations, predictions of potential fish
catches based on this method remain uncertain and unreliable. Nevertheless,
equation 5.2 provides relative valúes that are useful for comparing
production in different marine areas.

QUESTION 5.6 ]f the primary producto vity of a Coastal urea is
300 g C m -2 yr _l and herring (whieh feed on zooplankíon) are the principal
fishery. what would be the expected annual máximum yield of herring (in
terms of g C m _2 h given an average ecológica! effieieney of I0%?

Table 5.1 couples general valúes for primary productivity (from Section 3.6)
in the three major pelagic habitats (Figure 5.3) with numbers of trophic
levels, and leads to the conclusión that upwelling regions should produce by
far the highest numbers of fish (or whales) per unit area, and the open ocean
the fewest. Even correcting for percent ocean area, the small upwelling areas
should produce four times more fish than Coastal areas. In fact, upwelling
areas provide a significant fraction of the present world fish catch. Remember
too that there is a major economic advantage in catching fish in upwelling
and coastal areas because large numbers can be harvested within 30-80 km
of the coastline. In contrast, the expense of commercial harvesting is much
higher in the open ocean because the stocks are dispersed over a vast región.

5.2 FOOD WEBS

In reality, the concept of a food chain is a theoretical convenience and an
attempt to reduce a complex natural system to simple dimensions. There are
seldom simple linear food chains in the sea. Practically all species of
organisms may be eaten by more than one predatory species, and most
animáis eat more than one species of food. The energy system is more
accurately portrayed as a food web with múltiple and shifting interactions
between the organisms involved. Many species do not conveniently fit into
the conventional trophic levels. Some species are omnivorous, feeding on
both phytoplankton and zooplankíon. Some also feed on detritus, the
organic debris of faecal material, plant and animal fragments, crustacean
molts, and abandoned larvacean houses and pteropod feeding webs. Some
species change diets (and trophic levels) as they grow, or as the relative
abundance of different food items changes. Still other species are parasites
and obtain their energy from their hosts, and cannibalism is not uncommon
within many marine species. Further, the benthic food chain is also linked to
the pelagic production, as is illustrated in the continental shelf habitat in
Figure 5.3. Some benthic species (e.g. barnacles, mussels) feed directly on
phyto- or zooplankton, and other benthos are indirectly dependent on the
pelagic production.

There are typically fewer species in high latitude communities, and polar
marine food webs tend to be simpler than those of other localities. For this
reason, we have chosen to present a schematic depiction of the food web of
the Antarctic Ocean in Figure 5.4. Note that in this area, as well as in the
Arctic Ocean, there are two basic types of primary producers: the pelagic
phytoplankton and the algae which live within the ice. The latter, called
epontic algae, are generally benthic species that are adapted for the low light
intensities present at the undersurface of the ice. The abundant krill form the
central point of the Antarctic food web as they are the dominant herbivores,

118

Figure 5.4 A diagrammatic representation of the food web in the Antarctic Ocean.

and they are an important food source for several species of carnivorous
zooplankton, pelagic fish and squid, as well as for plankton-feeding baleen
whales, seáis, and seabirds. The abundance of krill may best be illustrated by
pointing out that a single blue whale may consume up to eight tonnes (or
more than 40 million individuáis) of euphausiids daily during summer
months. When several species rely heavily on a single food, as illustrated in
Figure 5.4, competition for food may develop between the different
predators if their shared food becomes a limiting resource. The decimation
of Antarctic baleen whales by commercial whaling has demonstrated how
the flow of energy through a food web may be altered when a competing
species is eliminated or reduced. Table 5.2 presents the relative amounts of
krill consumption by different predators before and after depletion of whale
stocks. A large decline in baleen whale biomass made more of the euphausiid
biomass available to competing species, and consequently increased the
populations of Antarctic seáis and birds by about a factor of three.

Competitive relationships among species may also diminish the biomass of
top-level commercially-fished species. Larval fish, for example, may
compete with chaetognaths, jellyfish, ctenophores, and other invertebrate
carnivorous zooplankton for copepods or other prey. Consequently, only a

119

Table 5.2 Estimated changes in pattems of consumption of Antarctic krill by the
major groups of predators, 1900-84._

Predator

1900

Annual krill consumption
(tonnes x 10 6 )

1984

Baleen whales

190

Seáis

130

Penguins

130

Fish

100

Squid

100

Total

470

certain fraction of the prey (and ultimately of the primary production) is
converted to fish stocks. Competition for food and the resultant loss of
energy to higher trophic levels is illustrated schematically in Figure 5.5. The
same planktonic invertebrate predators also may eat the meroplanktonic
larvae of the benthos, and thus they can decrease the production of shellfish,
such as mussels and clams, as well as of noncommercial benthos.

Although food webs are more realistic than food chains, they are more
difficult to quantify in ecological terms. Few marine Systems have been well
enough studied to attempt an energy budget analysis, in which the initial,
measured, energy input of primary production is channelled into different
trophic levels and pathways of the food web. The North Sea, however, has
been intensively studied because of its long-time importance as a fishing
región, its relatively small area, and its proximity to centres of marine
research. Figure 5.6 outlines a quantitative analysis of energy flow through a
food web of the North Sea based on major groups of organisms, rather than
individual species.

Food Chain

Food Web

Figure 5.5 A comparison of a hypothetical
marine food Chain and food web. Both begin
with 100 arbitran/ units of phytoplankton
primary production. The food Chain produces
0.2 units of fish from this primary production,
but the food web produces only one-half this
amount of fish. In the food web, two
carnivorous species (A and B) compete for the
supply of herbivorous zooplankton. Half of the
herbivores are consumed by Species A and half
by Species B. Fish do not eat Species B, so
their principal food supply is less by 50%.

120

Figure 5.6 A North Sea food web. Numbers
reler to annual production in g C m~ 2 .

The food web in Figure 5.6 is based on a primary productivity of
90 g C m~ 2 yr' 1 , and it assumes (probably unrealistically) that the
zooplankton consume all of the phytoplankton and excrete about 30% of
their food as faeces. The pelagic herbivore production of 17 g C m~ 2 yr -1
has been estimated from field and experimental studies on the copepod
Calanus finmarchicus, which is the dominant planktonic herbivore in the
System. The production valúes for the next trophic level come from
assuming that 50% of the herbivore production is eaten by planktonic
invertebrate camivores (such as chaetognaths and ctenophores) and 50% by
fish. This equal allocation of energy is assumed, however, to result in lower
production valúes for fish (even though they may additionally consume some
invertebrate camivores) because they have higher energy demands and burn
a higher percentage of their food in respiration than do invertebrates. The
partitioning of energy through the sinking of faeces, their decomposition by
bacteria, and the incorporation of bacterial production into benthic
invertebrates has been based on similar assumptions. The predicted yield to
humans in terms of fish catch is less than 0.7 g C m~ 2 yr' 1 , which is about
0.8% of the plant production.

Although the valúes of production in Figure 5.6 are extremely tentative and
based on numerous assumptions, the model provides a scheme that links
pelagic and benthic production and is an attempt to quantify a complex
ecological system. Despite the difficulties, there are many potential rewards
in studying food webs. Such studies allow us to examine the interactions of
nutrient input, primary production, and secondary production, and to
discover the determinants of production. They also can provide answers as to
why there are particular patterns of species associations and of energy flow,
and how these persist over time. It is also important to know how food webs
will react to perturbations, such as pollution and commercial harvesting of
high trophic levels.

121

5.2.1 THE MICROBIAL LOOP

The regeneration of nutrients in the sea is a vital part of the interaction
between higher and lower trophic levels. This is accomplished by bacteria
and planktonic protozoans interacting in a microbial loop that is coupled
with the classic food chain formed by phytoplankton-zooplankton-fish
(Figure 5.7). Particulate detritus formed through natural mortality of phyto-
and zooplankton and nekton, or through the production of faecal pellets and
structures such as crustacean molts, abandoned pteropod feeding webs or
larvacean houses, is decomposed by bacteria which utilize the energy-rich
detritus for growth. Bacteria also can utilize soluble organic materials
released by the physiological processes of animal excretion and
phytoplankton exudation, thereby efficiently converting dissolved nutrients
into particulate biomass. Thus the microbial loop is of particular importance
in increasing food chain efficiency through utilization of both the very
smallest size fractions of particulate organic material (POM), as well as of
the dissolved organic matter (DOM) which is usually measured as dissolved
organic carbón (DOC).

The number of bacteria in the euphotic zone of the oceans is generally
around 5 x 10 6 mi -1 . They may sometimes increase to 10 8 mi -1 in the
presence of adequate nutritive materials and in the absence of bacterial
grazers. In deep ocean waters, bacterial numbers may be less than 10 3 mi -1 .
The number of bacteria in the sea is generally controlled through predation
by nanoplankton, especially by various protozoans, but a few larger
zooplankton (e.g. larvaceans) are also capable of capturing and consuming

Figure 5.7 A schematic illustration showing the coupling of the peiagic grazing food chain
(phytoplankton to piscivorous fish) and the microbial loop (bacteria and protozoans). Dashed
arrows indícate the release of dissolved organic material (DOC) as metabolic by-products.
The DOC is utilized as a source of carbón by heterotrophic bacteria. The bacteria are
consumed by protozoans, which in turn are eaten by larger zooplankton.

122

10 5 r.

~l lililí- 1 I 1 nuil_I » 1 I _ I 1 i lililí I I ni

0.03 0.1 1.0 10 100

chlorophyll a{\x g I' 1 )

Figure 5.8 The relationship between
chlorophyll a and bacterial abundance in the
euphotic zone. Data are from •, central North
Pacific gyre; o, Southern California Bight and
Santa Monica Basin; and various additional
marine (a) and freshwater (□) locations.

bacteria (see Section 4.2). Among the nanoplankton, the zooflagellates are
particularly voracious in their consumption of bacteria. Heterotrophic
zooflagellates are usually present in concentrations of about 10 3 mi- 1 .
However, when bacterial numbers start to increase, the zooflagellates quickly
respond by consuming more bacteria and multiplying at a rate that tends to
prevent very large increases in bacterial standing stock. Small bactivorous
zooplankton are important links in transferring bacterial production to higher
trophic levels as they, in turn, form a source of food for larger organisms,
particularly for filter-feeding crustaceans. In general, filter-feeders like
copepods and euphausiids are incapable of feeding directly on bacteria
because their filtering appendages are too coarse to retain such small
particles.

0UESTI0N 5.7 In addiüon to heterotrophic zooflagellates and larvaceans. what
other zooplankton are eapable of feeding on bacteria? (Rder to Section 4.2. )

The general pathways of the recycling processes described above are shown
in Figure 5.7, where it is apparent that bacterial activity in the sea is closely
linked with marine food webs. Changes in the phytoplankton standing stock,
in particular, are often closely accompanied in time and distance by changes
in bacterial biomass. This is illustrated in Figure 5.8 which demonstrates that
as the chlorophyll concentration increases from about 0.5 ¡xg to 100 ¡xg l -1 ,
bacterial densities increase from about 10 6 mi -1 to 3 x 10 7 mi -1 . During
exponential growth of the phytoplankton, bacteria can live off dissolved
organic metabolites (exudates) that are released as part of the metabolic
processes of phytoplankton growth. At the end of a phytoplankton bloom,
when the algae enter a senescent stage, there is an accumulation of
phytodetritus (i.e. nonliving particulate matter derived from phytoplankton)
and an increased release of dissolved metabolites. It is particularly at this
time that the bacteria can utilize these energy sources to multiply and
produce a sharp pulse (or bloom) that follows the phytoplankton bloom.

Thus the food web of températe seas may shift seasonally from one that is
based on high nutrients, diatoms, and filter-feeding copepods, to one that is
dominated by the microbial loop and bactivorous zooplankton. A similar
relationship between phytoplankton and bacteria influences the vertical
distribution of bacterioplankton. Máximum numbers of bacteria generally
occur at the pycnocline, where phytodetritus accumulates by sinking from
the overlying euphotic zone. There, decomposition by bacteria contributes to
the formation of oxygen minimum layers in stable waters (see Section 4.4).
In general, it is estimated that bacteria, by using phytodetritus or dissolved
organic exudates for their growth, may utilize up to 50% or more of the
carbón fixed by photosynthesis.

Note in Figure 5.8, however, that the relationship between increasing
phytoplankton and increasing bacterial numbers holds least well at very low
chlorophyll concentrations (<0.5 ¡xg l -1 ), where bacteria are more numerous
than expected. This indicates that, in very oligotrophic waters, bacteria
constitute the dominant biomass of the microflora, and their numbers are
independent of the very small amount of phytoplankton. In waters where
nutrient concentrations are very low and limiting, there may be competition
between bacteria and phytoplankton for essential elements. In this
circumstance, predation on the bacteria by protozoans may influence the
outcome of the competition.

123

QUESTIQN 5.8 In Figure 5.8, why are ihe valúes for chlorophylt a and
bacieriaJ abundance lowesi in the central North Pacific gyre and highest in
fres h w ater I oca t i o n s ?

Marine viruses are the smallest and most abundant organisms in the sea,
with concentrations ranging between about 10 3 and 10 9 mi -1 , yet their role
in the microbial loop and generally in marine ecosystems remains highly
speculative. There are significantly more viruses in near-surface waters
compared with deeper layers, suggesting a coupling of viral particles with
other upper-ocean biological processes. It is known that viral pathogens can
infect marine bacteria and a variety of phytoplankton including diatoms and
Cyanobacteria, and experimental work suggests that viral infection may
affect the species composition of the phytoplankton community and
significantly reduce primary productivity.

Studies of the microbial loop are relatively new in the Science of biological
oceanography. They have been hampered by the very small size of the
microbes and protozoans and associated difficulties in collection,
preservation, and identification. There is a need to understand the impact of
this cycle on primary production in terms of nutrient competition and
nutrient recycling, and on secondary production in terms of providing a link
between bacterial production and its consumption by higher trophic levels.

5.3 MEASURING SECONDARY PRODUCTION

5.3.1 FIELO STUDIES

It is possible to obtain reasonable estimates of the amount of primary
production in different marine areas (Section 3.2.1), and catch statistics of
commercially fished species provide a minimum valué of production at the
other end of food chains. Information about secondary production in the
intermediary trophic levels and for noncommercial top-level predators (i.e.
jellyfish, ctenophores, Trash fislT) is much more difficult to obtain. There
are, however, compelling reasons to tackle this problem. In some
circumstances, primary production may not be a good indicator of
production in higher trophic levels. For example, in highly eutrophic systems
(see Section 3.4), the growth of phytoplankton may greatly exceed what can
be consumed by herbivores, or the phytoplankton species which becomes
dominant under these conditions may not be a suitable food source for
herbivores; in either case, much of the primary production may enter the
microbial-detritus circuit instead of the classic pelagic food chain. Relying
entirely on fish catch statistics to provide valúes of secondary production in
top trophic levels leads to underestimates, because it omits production of all
the competing, unharvested species.

Secondary production can be estimated from field data. The production of a
population of zooplankton is defined as the total amount of new biomass of
zooplankton produced in a unit of time, regardless of whether or not it
survives to the end of the time period. In this definition, biomass (B) is:

B = Xxw (5.3)

where X is the number of individuáis in the population and vv is the mean
weight of an individual. It then follows that production (P t ), during a time

124

interval from t\ to t 2 , can be expressed as:

VVi + w 2

P t = (X i -X 2 ) — 2 + (5.4)

where the subscripts 1 and 2 indícate valúes obtained at time t\ or time t 2 ,
respectively. The expression (#2 — # 1 ) represents the increase in biomass
observed during the time interval; the remainder of the equation (i.e. the
decrease in population number times the average weight of an individual)
represents an estímate of the biomass produced, but then lost through
predation or water movements.

Ideally, one would wish to follow changes over time in numbers and growth
under natural conditions in a single cohort of a population, a cohort being
one identifiable generation of progeny of a species. However, these
conditions can seldom be met in the sea and, in any event, it is usually
impossible to follow and sample the same water mass for a period of time
long enough to obtain meaningful measures of growth. The best that can
usually be achieved is to attempt to follow changes in relative numbers and
weights of distinctive life stages of abundant species. Because many copepod
species are dominant members of the plankton and have easily identifiable
age classes, this group of crustaceans is often selected for production
measurements, and the example given below is for a copepod species
producing one generation per year.

Figure 5.9 presents a hypothetical representation of the numbers of
individuáis in different, successive developmental stages of a copepod,
ranging from newly hatched nauplii through copepodite stages I, III, and V
(see Section 4.2). Note that the numbers (X) change with time due to a
number of natural processes including mortality, aggregation, or water
exchange. In order to calcúlate production, we also need to calcúlate changes
in weight, or growth. This can be done if the time interval between the

Figure 5.9 A hypothetical representation of changes in the numbers of individuáis in
selected successive developmental stages of a copepod having one generation per year.

Some stages are difficult to distinguish from each other, so for that reason all naupliar
stages (NI-NVI) are lumped together and only copepodite stages Cl, Clll and CV are
considered here. (Refer to Section 4.2 for a discussion of copepod life cycles.) Wt and w 2
indícate average weights of copepodite stages Cl and Clll, respectively.

time

125

occurrence of máximum numbers of successive developmental stages is
known, and the average weight of each stage is determined in the laboratory.
In this example, there is a duration of 44 days between copepodite stage I
and copepodite stage III, and a change in average weight from 0.15 mg to
0.60 mg between these respective stages. Assuming that there were, on
average, 80 stage I copepodites m -2 and 30 stage III copepodites m -2 , we
now have the information needed to calcúlate production.

QUESTK0N 5.9 Given i he Information above, and mmg equation 5.4. whai
was the average produeiion per day of the copepod population during the
44-day period between peak numbers of stage 1 and lll copepodites?

It is important to note that the valué of P t for any population will change
with time because zooplankton grow at different rates during their Ufe
eyeles. Production will generally be positive during periods of máximum
growth; in températe regions, P t would be highest in spring when food is
abundant and most of the zooplankton are young. The valué for P t may be
negative in winter months when food concentrations are low and animáis
cease growing, or even lose weight. Because of these natural changes in P t ,
growth increments need to be determined for a number of points in the life
history of a species in order to calcúlate annual secondary production. Thus,
annual secondary production (P t ) becomes a summation of production
calculated for successive time intervals (P t i, Pa , etc.), so that:

Pt — Pt\ + Pt2 + Pt3 • • • Pti' (5.5)

The method described above may only be applied to species with distinctive
life stages, and it is only practicable in regions where young are produced
seasonally. Particularly in warm waters, many marine animáis (including
copepods and other Crustácea) reproduce more or less continuously, and thus
there is a continuous input of young and a mixture of age and size classes.
Further, many planktonic species do not have distinctly different life stages,
and size or weight may not necessarily be good indicators of age. A good
example of this is found in the thecosomatous pteropods (refer to
Section 4.2); very young individuáis quickly produce an adult-sized shell,
but the body grows only gradually to occupy the entire inner space. Whereas
crustaceans have a determínate growth pattern in which growth in each
stage is limited by size of the exoskeleton, many of the other zooplankton
have indeterminate growth and are capable of more or less continual
growth during favourable conditions, or they shrink (lose biomass) during
periods with low food concentrations. Often the attendant difficulties of
working with population data collected from the field lead researchers to
work with populations or individuáis under experimental confinement, where
conditions can be controlled and where individuáis can be studied over
prolonged periods of time. Various experimental approaches to secondary
production studies are discussed in the following section.

5.3.2 EXPERIMENTAL BI0L0GICAL 0CEAN0GRAPHY

Because the oceans are so vast and the waters are in continual motion, and
because of the reasons outlined at the end of the previous section, it is
necessary to study some biological oceanographic processes under
experimental conditions.

126

Many options are open to the experimentalist and some examples of
different approaches are discussed below. In general, however, a choice can
be made between:

(1) laboratory-scale experiments, which tend to study individual
organisms in relatively small volumes of water;

(2) enclosed ecosystem experiments, which are carried out in very large
containers of natural seawater in order to test the interactions of several
trophic levels and their responses to perturbations; and

(3) Computer model simulations, in which complex biological processes
and ecological interactions can be studied on a Computer, including the
influences of physical and Chemical environmental parameters on
biological processes occurring in the sea.

No single method of study is likely to provide all the answers required, and
all experimental and observational approaches, from laboratory test-tube
studies to satellite data-gathering, have both advantages and limitations to
the type of results that are generated. Figure 5.10 illustrates schematically
how the three experimental approaches outlined above can interact with field
studies to lead to a better understanding of natural events. Data gathered
from the field and from experimental studies can be fed into mathematical
Computer models which attempt to intégrate this information and simúlate
real conditions and events. These experimental options apply to studying
problems in both pelagic and benthic marine ecology, but the examples
discussed below concéntrate on issues in planktonic ecology.

Laboratory experiments

Experiments in the laboratory can be conducted to determine food
requirements of zooplankton and transfer efficiencies between trophic levels.
The majority of such experiments have been carried out using crustaceans,
principally copepods, because they can be easily captured in large numbers
and with little or no damage, and they are also amenable to laboratory
culture. The majority of experiments have also been done with herbivorous
species, partly because their phytoplankton food can be cultured easily. A

Figure 5.10 The interaction of field studies,
experimental studies, and Computer modelling
in biological oceanographic research.

127

few carnivorous zooplankton, including certain chaetognaths and
gymnosomes, have been utilized for experimental research but, in these
cases, their zooplankton prey must also be amenable to culture or,
alternatively, the prey must repeatedly be collected fresh and with minimal
damage from the sea. The same principies and equations, using modified
techniques, can be employed to study production in benthic animáis.

Experimental studies of feeding and secondary production are based on the
premise that only a certain fraction of the energy ingested in food can be
utilized for production. The remaining fractions are expended in respiration
or excretion, or are never utilized and pass through the animal in faecal
material by the process known as egestion. The fractionation and utilization
of energy can be expressed as:

G = R-E -U-T (5.6)

where growth (G) is a measure of secondary production; R is the ration of
ingested food; E is egested faecal material; U refers to excretory products
(e.g. ammonia, urea); and T represents respiration. The units in this energy
balance equation are given in joules or calones per unit weight. Excretory
products are usually considered as a negligible fraction of the equation, and
the equation can be simplified and rewritten as:

AR — T G (5.7)

where R , T and G are as defined above, and A is a constant relating the
proportion of food assimilated (or actually utilized) to the amount consumed.
AR is thus referred to as the assimilated ration. The rationale for establishing
equations 5.6 and 5.7 is presented schematically in Figure 5.11.

Figure 5.11 A schematic división of the losses
and uses of the energy contained in consumed
food.

Feeding experiments usually involve introducing from one to several tens of
animáis (depending on their size) into incubation beakers filled with a
measured volume of seawater containing a known concentration of food
partióles. Controls are established in bottles containing food but no animáis;
these will show any changes in food concentration that occur independently
of grazing or predation and will allow an assessment of error in counting
food organisms. In the case of grazing experiments with filter-feeding
herbivores, culture bottles are kept dark to minimize growth of the
phytoplankton. Usually it is necessary to introduce a mechanical means of
keeping cells in suspensión and randomly distributed in the water. After
containers have been incubated for a measured time interval, the food
concentration is again determined. This can be done visually with a
microscope, or electronic particle counters may be used if the particles are
small and of appropriate shape.

There are a number of ways to calcúlate the grazing rate (number of algal
cells eaten per herbivore per hour or day) or the predation rate (number of
prey animáis eaten per carnivore per hour or day). Both of these rates
become more meaningful for comparative purposes if they are converted to
ingestión rate, which is the weight or energy content of food ingested per
animal per hour or day; in this case, weight can be expressed in terms of dry
weight, organic matter, carbón, or nitrogen. Many factors affect feeding rates
including temperature, type of food, and concentration of food. There is
usually a direct correlation between the amount of food eaten and the food
concentration, the relationship being expressed as:

R = KmaxO - e~ kp )

(5.8)

128

where R is the radon of food ingested at a food or prey density /?, R m . dX is
the máximum radon taken at sadation, and A: is a grazing constant linking
food concentradon and ingestión. Thus the radon increases with food
concentration to some máximum level, as shown in Figure 5.12.

In order to establish the proportion of food that is actually digested in the gut
and assimilated by an animal (A/? in equation 5.7), an assimilation efficiency
is calculated by comparing the ingested food with a quantitative measure of
faeces produced. Thus, assimilation efficiency (A) can be calculated from:

A =

R-E

x 100%

(5.9)

where R is the amount of food (radon) ingested and E the amount of faeces
produced. (Note that in equation 5.7, A is the assimilation efficiency given in
decimal form.) Although assimilation efficiency varies with type of food, age
of an animal, and other factors, it is generally high ( ca . 80— > 90%) for
carnivorous animáis, and somewhat lower (ca. 50-80%) for herbivores.
Detritus feeders have the lowest assimilation efficiencies, usually being less
than 40%.

Why are assimilation efficiencies difierent in carnivores* herbivores, and
detri livores?

Assimilation is high in camivores because of the similar biochemical
composition of the prey and predator. It is lower in herbivores because of
the greater difficulty in digesting plant food, particularly carbohydrates and
especially cellulose. Assimilation efficiency is lowest in animáis that feed on
detritus because much of the food they consume, such as skeletal
components, is indigestible.

Figure 5.12 The relationship between the
amount of food ingested (R) and the
concentration of food available (/?). Note that R
increases up to a certain food concentration,
then remains at the same level (/? max ) with
further increases in food.

Valúes for respiration losses (T in equations 5.6 and 5.7) can be determined
in the laboratory. Bottles with and without experimental animáis are
prepared simultaneously with known volumes of seawater. Changes in
dissolved oxygen concentration after an incubation period are detected by
Chemical methods or oxygen electrodes, and are attributed to respiration of
the animáis. Respiration rates are directly correlated with the environmental
temperature, but they are inversely correlated with size of an animal, being
higher per unit weight in smaller animáis. In general, 40-85% of the
assimilated food (AR) will be utilized for metabolic maintenance.

Hovv would difieren! respective respira! ion mies affect the daily food raí ion
i>f sma11 zooplankton compared with 1 urge zoopIankton?

Because small zooplankton have higher respiration rates per unit body
weight, they require much higher food intake relative to body weight. Very
small zooplankton (e.g. crustacean nauplii) may require and ingest food
equivalent to more than 100% of their body weight per day. In contrast,
large zooplankton (e.g. adult euphausiids) may eat the equivalent of only
about 20% of their body weight per day.

Referring to equation 5.7, growth or production (G) of zooplankton can be
calculated indirectly from valúes obtained for ingestión rates (/?),
assimilation efficiency (A), and metabolic losses (T). In some circumstances,
growth can be measured directly in laboratory experiments by measuring or
weighing experimental animáis at successive time intervals. There are two
expressions that link the growth (G = AW/At) of an animal with the

129

amount of food ingested ( R ) by that animal. One is known as the gross
growth efficiency, K\, which is expressed as:

K\ = G/Rx 100%. (5.10)

The other expression is the net growth efficiency, K 2 , which is a ratio of
growth (G) to assimilated ration (Afí), so that:

K 2 = G/ARx 100%. (5.11)

QUESTIQN 5,1 B The gymnosome Clione limad na, feeding orí the shelled
pteropod Umaana hethiruL inereased in dry weight by un average of 5.0 mg
per month, while consuming an average of 7.5 mg dry weight of prey.

Clione assimilates its prey with 90% efficiency, (a) What is the gross growth
efficiency for Clione? (b) What is the valué of K 2 for ibis gymnosome?

Temperature and food concentration will influence both K\ and K 2 valúes,
and both valúes change with age of an animal. The number of eggs produced
by experimental females may also be included as parí of the total production
in energy balance equations and, in general, fecundity increases with
inereased food ration. The efficiency at which assimilated food is converted
into growth or progeny (K 2 ) usually ranges between 30% and 80% for
zooplankton and fish. This is much higher than growth efficiency in
terrestrial mammals, which tends to be on the order of 2-5%. Some of the
difference arises because poikilothermic animáis (planktonic invertebrates,
fish) have lower metabolic costs than warm-blooded mammals. As well,
terrestrial mammals expend energy in fighting the effeets of gravity, whereas
pelagic species are buoyed up in their seawater environment.

Experimental studies of energy requirements and energy partitioning as
described above provide a more realistic basis for formulating ecological
principies that govern food chains in the sea. These types of experiments
permit us to judge the levels of food concentrations required to produce a
certain number of animáis at different trophic levels, at least under the
conditions maintained in the laboratory. Conversely, knowing the amount of
primary productivity in any particular marine región, and applying energy
budget valúes obtained from the laboratory, permits a better prediction of the
amount of secondary production that might be expected under those
conditions.

The laboratory approach to establishing energy budgets in zooplankton has
given us useful comparative valúes for different types of animáis, and has
allowed an assessment of transfer efficiencies based on ecological trophic
theory and field surveys. Nevertheless, it is important to keep in mind that
only some species are amenable to laboratory culture; the conditions are
always artificial: and the results must be extrapolated to natural situations,
including natural foods and changing environmental conditions.

Enclosed experimental ecosystems

An enclosed experimental ecosystem is a system in which a large volume of
natural seawater is artificially enclosed in order to study populations of
phytoplankton and zooplankton over time. This alleviates the problem of
studying plankton populations at sea, where they are being continually
displaced from one location to another by water movement. If the
experimental system is large enough, it may be possible to study the

130

interactions of several trophic levels, including such planktivorous species as
ctenophores and fish.

Several conditions must be met in setting up an enclosure. In order to contain
sufficient plankton and a reasonable semblance of the natural environment,
the volume of seawater captured must usually be between 100 and 1000 m 3 .
It must be enclosed in such a way that natural sunlight can penétrate the
water column. The depth profile of the water column in terms of temperatura,
nutrients, and salinity must also be preserved, and care must be taken that no
toxic substances are accidentally introduced. These conditions can usually be
achieved by using moored, transparent containers made of nontoxic material.

Figure 5.13 illustrates enclosures that were moored in a fjord in British
Columbia, Cañada, and used in studies known as the Controlled Ecosystem
Pollution Experiments (CEPEX). Each 30-m deep container enclosed about
1300 m 3 of seawater. The purpose of the programme was to test the effects
of small traces of pollutants on the ecology of the plankton community and
on young fish. The employment of several enclosures permitted the addition
of different types of pollutants in differing concentrations, as well as
providing a control enclosure that could be compared with conditions in the
surrounding, unenclosed water. Experiments were conducted with traces of
copper (10-50 ppb), mercury (1-10 ppb), petroleum hydrocarbons
(10-100 ppb), and several other potential pollutants that can be found in
seawater, particularly in Coastal areas.

Figure 5.13 An artist’s illustration of
controlled experimental ecosystem enclosures
floating in the sea. Each polyethylene container
is 30 m deep and holds about 1300 m 3 of
seawater. The scuba divers are drawn to
approximate scale.

Figure 5.14 provides an example of the results that were obtained in one
experiment. At the start, diatoms ( Chaetoceros ) were the dominant primary
producers in all enclosures, and they remained dominant in the control
containers (J and K) throughout the experimental period. In the experimental
ecosystems treated with copper (L and M), most of the diatoms died within
three weeks (Figure 5.14a) and were replaced by small photosynthetic
flagellates that began to increase after the first week following pollutant
addition (Figure 5.14b). This change in the structure of the primary
producers from large diatom chains ( ca . 500 (x m) to small flagellates of less
than 20 /im is analogous, but on a microscale, to a change in size of
terrestrial vegetation from trees to grass, and it had a profound influence on
the total ecology of the enclosures. Changes in the phytoplankton
community were accompanied by changes in the dominant species of the
zooplankton community and, after about one month, the copper-treated and
control containers had very different ecologies. This kind of result is almost
impossible to obtain from either laboratory-scale experiments (which are too
small) or from field experiments (where the water moves around too much).

QUESTI0N 5,11 Figure 5.14 shows that a small amount of copper ¡n seawater
caused a change in the primary producers, with long-chain diatoms being
replaced by small flagellates, What difference might this make to the rest of
the food chain in terms oí the type of dominant species and nurrtber of
trophic levels? (Refer to Section 5,1 for help ií necessary.)

Enclosed ecosystem experiments have been performed at many different
locations including in Loch Ewe in Scotland; at the Marine Ecosystem
Research Laboratory in Rhode Island, U.S.A.; and in Xiamen in the People’s
Republic of China. Not all of the experiments have involved pollutants.

Some have been designed to test the effects of natural perturbations, such as
changes in light levels and in amount of vertical mixing, on plankton

131

Figure 5.14 The results of experiments in enclosed experimental ecosystems. (a) The
percentage of long-chain diatoms (Chaetoceros) in the total phytoplankton. (b) The
percentage of small flagellates in the total phytoplankton. L and M, copper-treated
containers; J and K, control containers.

ecology. Such studies provide valuable insights into the influence of physical
variables on biological processes.

The disadvantages of using enclosed experimental Systems of this type
should also be mentioned. Perhaps the greatest drawback is the removal of
small-scale physical turbulence from enclosed waters. Mixing is an
important property of natural marine communities, and the damping out of
turbulence within the containers can lead to spurious results if the
experiments are continued for too long a time. Also, some caution has to be
exercised in applying results obtained in experiments in one area to other
locations. The waters of Loch Ewe, Scotland, for example, are not the same
(physically, chemically, or ecologically) as those in the South China Sea,
and similar experimental procedures may produce quite different results in
the two localities.

Computer simulations of marine ecosystems

Computer models provide a third way of generating knowledge about the
way marine ecosystems opérate. Data gathered from many sources (e.g. field
measurements, laboratory or controlled ecosystem enclosure experiments,
satellite imagery) are entered into mathematical models designed either to
simúlate natural events or to provide predictions of future events. The type
of model used depends on the questions being asked. A relatively simple
model can be used to predict an oxygen balance in a particular body of
water, but more complex models are required to examine trophic interactions
between several levels in marine food webs.

The usual approach in setting up Computer models that will examine
ecological issues is to formúlate a number of differential equations made up
from non-linear empirical relationships which connect various forcing
functions with ecotrophic structure (Figure 5.15). For example, the amount
of available light and the concentrations of nutrients would be considered
forcing functions in such equations; they set the constraints on production
in the system being considered. A model would also consider non-linear

132

Figure 5.15 Ecosystem Computer model
construction showing examples of forcing,
physiological, and phasing functions.

1. Forcing functions 2. Physiological functions

2.1. Primary productivity vs. light

2.2. Zooplankton grazing on phytoplankton

3. Phasing functions

3.1. Extinction coefficient on light

3.2. Temperature effect on growth

physiological functions, such as the reaction of light with phytoplankton
(see Section 3.3). In addition to these two types of functions, there are a
number of other environmental influences which modify both the forcing and
physiological functions. For example, changes in temperature will modify
most physiological functions, and the light extinction coefficient will modify
the forcing function of light. These modifying influences form a third type of
function; they are called phasing functions because they tend to speed up or
slow down the interactions between forcing and physiological functions.

The layout of a simple Computer model involving the interaction of different
trophic levels is shown in Figure 5.16. In this system, the growth rate of the
phytoplankton is govemed by light and nutrients, The zooplankton graze the
phytoplankton according to an equation in which the grazing rate is
dependent on the concentration of phytoplankton; the one planktivorous
species (a ctenophore) utilizes the zooplankton depending on the availability
of the prey. The model also ineludes a reeyeling loop in which bacteria
utilize dissolved organic material (DOC) lost from the phytoplankton and
retum part of it to the system via zooflagellates and microzooplankton. This
model can be run on a desk-top Computer, and an illustration of the kind of
results that one can obtain are shown in Figure 5.17.

GUESTIÜN 5.12 In the Computer model layout shown in Figure 5*16 which are
(a) the forcing functions and (b) the physiological functions?

Figure 5.17 shows the results of a Computer simulation designed to test the
effect of changing only one parameter, the extinction coefficient of light, on
the output of the model in terms of the amount of phytoplankton and
zooplankton produced. The amount of light was decreased by increasing the
extinction coefficient (see Section 2.1.2) from 0.2 to 0.3 and to 0.7 m" 1 . As
one might expect, the amount of phytoplankton decreases during an 8-day
period as the extinction coefficient increases. However, the standing stock of
zooplankton increases with a decrease in light intensity from extinction
valúes of 0.2 to 0.3 m -1 . This increase can be attributed to the fact that the
phytoplankton grow more slowly at an extinction valué of 0.3 m -1 than at
0.2 m _1 , and this in turn enables the slower-growing zooplankton to graze
more of the phytoplankton, which would otherwise sink out of the water
column as ungrazed phytodetritus. The very abrupt decline in the
phytoplankton at day 4 (when k = 0.2) is due to nutrients becoming
exhausted by the rapidly growing phytoplankton, and to the sinking of
accumulating phytodetritus. The maximal zooplankton growth at a light
intensity in between very bright and shady is a result of interactions within

133

Figure 5.16 A simple trophodynamic model
for use in Computer simulations of trophic
interactions. DOC, dissolved organic carbón.

the model, and could not be readily seen without the modelling approach.
The use of ecosystem models has indicated that many other trophic
relationships optimize at intermedíate environmental valúes.

QUESIIOH 5.13 In the computar model resuíis shown in Figure 5,17, why is
the pro Judión of zooplankton lowest vvhen k — 0.7?

Computer simulation models have both advantages, as described in the
example above, and disadvantages. They cannot replace the gathering of real
data. Efforts can be made with models to simúlate actual events. More often
than not, however, data generated by actual events are the result of so many
variables that it is statistically difficult to tune any one model to explain
them. Thus a model may more often be used to project different scenarios of
likely events. When compatibility with actual field data is required, a
biological trophodynamic Computer model must be coupled to a Computer
model of the physical environment. Such models have been produced, and
an example is the General Ecosystem Model for the Bristol Channel and

134

k=(U

Figure 5.17 A Computer model simulation
predicting the production of phytoplankton
(solid lines) and zooplankton (dashed lines)
from different light extinction coefficients (Ür),
(Standing stocks in relative units.)

3 4 5

days

Severn Estuary (known as GEMBASE) developed by the Institute for
Marine Environmental Research in Plymouth, England. In this Computer
model, an ecological submodel is operated in seven different physical
domains, and water exchanges between these subregions are determined by
two physical models describing water flow. Integrated water exchanges
produced from the physical models are then entered as input into the
slower-scaled ecological models.

5.4 A COMPARISON OF MARINE AND TERRESTRIAL
PRODUCTION OF ORGANIC MATERIAL

The preceding discussions of pelagic production and food webs have
highlighted unique ecological features of the marine habitat. You may
already have noted profound differences between marine and terrestrial
ecology. The follgwing points summarize differences in primary and
secondary production in these two major environments. You may wish to
compile your own list of other differences in physical features of the
contrasting environments and of general anatomical and behavioural
differences in marine and terrestrial organisms.

1 In the open ocean, the majority of the primary producers are microscopic
phytoplankton (Sargassum weed being one of the few exceptions). Although
macroscopic algae and sea grasses contribute to marine production in inshore
shallow areas, by far the greatest amount of marine primary production is
carried out by the small phytoplankton. In contrast, the great majority of
terrestrial primary producers are large, highly visible forms like grass and
trees.

2 The basic structure of phytoplankton and terrestrial vegetation is very
different. The small size of phytoplankton enhances flotation and also,
because of the surface to volume ratio, promotes uptake of nutrients directly
through the cell wall from the surrounding water. Terrestrial plants, in
contrast, require roots for anchoring and for nutrient uptake from the soil.
They also tend to develop trunks and branches to fight gravity and maximize
exposure to sunlight, and this requires the production of carbohydrates like
cellulose and lignin which confer strength and rigidity. Phytoplankton, in

135

contrast, do not require large amounts of structural carbohydrate and are
largely composed of protein.

3 Because of the small size and high protein content of the phytoplankton
and their relatively low abundance compared to herbivore numbers, a major
fraction of the marine primary production is usually consumed, digested, and
readily assimilated by marine herbivores. Comparadvely little of the pelagic
primary production goes directly into the decomposer cycle. This is not the
case in the terrestrial ecosystem, where much of the primary production is in
the form of inedible or indigestible structural components, such as cellulose
and lignin, that are contained in bark, tree trunks, and roots. Terrestrial
animáis rarely eat more than 5-15% of the total plant production, and they
consume a lot of material that is largely indigestible. Much of the terrestrial
photosynthetic production therefore enters indirectly into the food chain via
the decomposition cycle.

4 In the pelagic environment, primary productivity ranges from about 50 to
600 g C m~ 2 yr -1 . In comparison, terrestrial primary productivity varíes
from virtually zero in arid deserts and regions that are too coid for plants
(Antárctica) to máximum valúes of about 2400 g C m -2 yr -1 in grasslands,
and about 3500 g C m -2 yr -1 in tropical rainforests. Although primary
productivity of benthic marine plants in shallow areas may approach
terrestrial valúes, in general the primary production per unit area in the sea is
much lower than on our ‘green’ land.

5 A useful comparison can be made by examining the production to
biomass ( P/B ) ratio; this is the relationship between the total annual
production and the average biomass of living plant (or animal) material
present throughout the year. Although the total biomass of the small marine
phytoplankton may be relatively low, these algae are fast-growing.
Consequently the P/B ratio for phytoplankton is roughly 100-300 in the
marine pelagic ecosystem; this means that the phytoplankton biomass may
turn over 100-300 times during a year. P/B ratios measured over one year
are about an order of magnitude lower for marine zooplankton, and another
order of magnitude lower for fish. In contrast, terrestrial plants have a very
high total biomass (we can see this), but are generally slow-growing and
long-lived; further, a large fraction of the primary production of these plants
is used to maintain the respiration of that biomass. Therefore, terrestrial
vegetation has much lower P/B ratios of about 0.5-2.0.

6 Most of the animáis in the sea are cold-blooded (poikilothermic)
invertebrates and fish and therefore have much lower energy requirements
than terrestrial warm-blooded birds and mammals. As well, pelagic animáis
live in a buoyant fluid and use little energy in locomotion. In comparison,
mammals and birds (and even the larger poikilothermic insects) expend large
amounts of energy fighting gravity; walking, crawling, and flying are all
more energetically costly than swimming. This means that a relatively larger
fraction of ingested energy can be channelled toward growth and
reproduction in marine pelagic animáis. In fact, growth efficiency (the ratio
of biomass production to quantity of food eaten) tends to be an order of
magnitude higher in marine poikilothermic species. In contrast to land
mammals and birds, they also produce larger numbers of young and usually
expend no energy on parental care of the progeny. All of these features
contribute to a much higher secondary production in the sea compared with
that on land.

136

7 Whereas plants so evidently domínate the biomass on land, animáis form
the visually dominant group in the sea. Although the ocean contributes only
about 50% of the world’s plant production, it accounts for more than 50% of
its animal production.

5.5 MINERAL CYCLES

All elements that become incorporated in organic materials are eventually
recycled, but on different time scales. The process of transforming organic
materials back to inorganic forms of elements is generally referred to as
mineralization. It takes place throughout the water column as well as on the
bottom of the sea, where much of the detrital material from overlying waters
eventually accumulates. Recycling of minerals may take place relatively
rapidly (within a season) in the euphotic zone, or much more slowly (over
geological time) in the case of refractory materials which sink and
accumulate on the seabed.

Figure 5.18 illustrates some of the ways in which elements are cycled by
different groups of organisms. In the water column, where there is usually
plenty of oxygen, decomposition of organic material takes place via
oxidative degradation through the action of heterotrophic bacteria. Carbón
dioxide and nutrients are returned for re-utilization by the phytoplankton.

The cycle differs in anoxic areas where there is no free dissolved oxygen.
Anoxic conditions are present in subsurface sediments on the seafloor and in
a few special areas like the Black Sea, which is anoxic from about 200 m
depth to the bottom because water exchange and mixing with the adjacent
Mediterranean Sea are severely restricted by bottom topography. Under
anoxic conditions, bacterial degradation takes place by anaerobic bacteria
that utilize oxygen found in sulphate and nitrate radicáis. This type of
oxidation forms highly reduced compounds such as methane, hydrogen
sulphide, and ammonia. Since these compounds are high in Chemical energy,
another group of bacteria (the chemoautotrophs) can utilize this energy to
reduce carbón dioxide and make new organic material. The process of fixing
carbón from CO 2 into organic compounds by using energy derived from
oxidation of inorganic compounds (e.g. nitrite, ammonia, methane, sulphur
compounds) is called chemosynthesis.

Ecologically, the most important aspect of recycling in the sea is the rate at
which growth-limiting nutrients are recycled. Among the nutrients that can
be in short supply in the sea, nitrate (NO 3 - ), iron (bioavailable Fe),
phosphate (PC> 4 3 ~), and dissolved Silicon (Si(OH) 4 ) are most often found in
concentrations well below the half-saturation levels required for máximum
phytoplankton growth (refer to Section 3.4). Silica limitation affects
primarily those organisms that use this element to form skeletons; these
inelude the diatoms and silicoflagellates among the phytoplankton (see
Section 3.1), and the radiolarians among the zooplankton (refer to
Section 4.2). The Silicon cycle is relatively simple as it involves only
inorganic forms; organisms utilize dissolved Silicon to produce their
skeletons, and this skeletal material dissolves following death of the
organisms. The eyeling of phosphorus is also relatively simple in a Chemical
perspective; at the usual alkaline pH of seawater, organic phosphate is
relatively easily hydrolysed back to inorganic phosphate which is then
available again for uptake by phytoplankton. Because phosphorus eyeles
rapidly through the food chain, it is seldom limiting in the marine

137

Figure 5.18 Marine organisms derive energy from light (photoautotrophs), from inorganic
compounds high in Chemical energy (chemoautotrophs), from organic carbón compounds
(heterotrophs) or, in the case of a few bacteria, from a combination of these processes.
Each of these sources of energy may become limited at some time or place in marine
habitáis. The supply of organic carbón compounds is largely limited by their rate of
production from chemoautotrophs and photoautotrophs. Photoautotrophs are limited by the
amount of light. Chemoautotrophs, which may be either aerobic or anaerobio species, are
limited by the amount of highly reduced inorganic compounds derived from the metabolism
of anaerobio heterotrophs. Thus each process is essentially dependent on the next process
to recycle material through the entire system.

environment. Compared with Silicon and phosphorus, the recycling of
nitrogen, however, is a more complex process.

5.5.1 NITROGEN

The marine nitrogen cycle (Figure 5.19) is complex because nitrogen in the
sea occurs in many forms that are not easily converted from one to another.
These inelude dissolved molecular nitrogen (N 2 ) and the ionic forms of
ammonia (NH 4 + ), nitrite (NO 2 - ) and nitrate (NC> 3 ~), as well as organic
compounds such as urea (CCKNF^h)- The dominant form of nitrogen in the
ocean is the nitrate ion, and it is often this form that is taken up by
phytoplankton, although many species can also utilize nitrite or ammonia. A
few phytoplankton species can also take up some small molecules of organic
nitrogen, such as amino acids and urea. The rate at which nitrogen in a
suitable State is made available for phytoplankton may limit primary
production in oligotrophic waters throughout the year and in températe

138

waters during the summer. Remember too (from Section 3.4) that iron is
necessary for the formation of reducíase enzymes that are used in the
conversión of nitrite and nitrate into ammonium, and this is used to make
amino acids. If iron is present in limiting concentrations, then even an
abundance of nitrate will not promote máximum phytoplankton production.

Regeneration of nitrogen in the water column results from bacterial activities
and excretion by marine animáis, especially the excretion of ammonia by
zooplankton. As illustrated in Figure 5.19, the oxidation of ammonia to
nitrite and then to nitrate is referred to as nitrification; the bacteria that
mediate this change in Chemical State are called nitrifying bacteria. The
reverse process of forming reduced nitrogen compounds from nitrate occurs
mostly in anoxic sediments and is called denitrification; these changes are
carried out by denitrifying bacteria. The nitrogen cycle also involves
nitrogen-fixation, in which dissolved nitrogen gas is converted to organic
nitrogen compounds; this process can be carried out by only a few
phytoplankton, notably some Cyanobacteria. Dissolved organic nitrogen
(DON) and particulate organic nitrogen (PON) both serve as nutrients for
bacterial growth. Bacteria break down proteins to amino acids and ammonia,
and the latter is oxidized in the nitrification process. The eventual release of
dissolved inorganic nitrogen (DIN) makes these forms available again for
uptake by the phytoplankton. The various types of bacteria involved in this
cycling can themselves serve as a direct source of food for some nano- and
microzooplankton.

An important aspect of the marine nitrogen cycle concerns the source of the
nitrogen used in primary production. Some fraction of the primary
production is derived from nitrogen recycled from organic matter within the
euphotic zone; another fraction is derived from new nitrogen which comes
from sources outside the euphotic zone (see Figures 5.19 and 5.20). New
nitrogen is primarily nitrate entering the euphotic zone from below the

Figure 5.19 The nitrogen cycle in the euphotic zone of the sea. The diagram illustrates the
recycling of nitrogen that takes place within the euphotic zone and above the nutricline, as
well as the ¡nput of ‘new’ nitrogen upwelled from deeper water. Note the interrelationships
between DIN (dissolved inorganic nitrogen), PON (particulate organic nitrogen), and DON
(dissolved organic nitrogen). The nutricline is where there is a rapid change in the
concentration of a nutrient with depth.

atmospheric exchange

DIN runoff

sinking

tota! phytoplankton production
(gC m^yr 1 )

total phytoplankton production
(g C m ' 2 yr 1 )

Figure 5.21 (a) Phytoplankton new production

as a function of total (regenerated + new)
annual phytoplankton production in different
marine environments ranging from oligotrophic
to eutrophic.

(b) Total carnivorous fish plus squid biomass
production as a function of total phytoplankton
production in marine environments ranging
from oligotrophic to eutrophic. The blue line
indicates the total amount of fish production
that results from new production only; this is a
sustainable yield because the nitrogen removed
in the harvest will be replaced through
upwelling of nitrate.

139

N 2 other N
fixation inputs

Euphotic

Zone

Figure 5.20 A comparison of production generated by recycled nitrogen and by new
nitrogen. Regenerated production results only from the reduced nitrogen supplied within the
euphotic zone by the excretion of organisms. New production depends on nitrogen supplied
from outside the euphotic zone, of which the dominant source is the nitrate moving upward
from below the nutricline. In a steady State, upwelled nitrogen is balanced by the downward
sinking of nitrogen bound in sedimenting particles. On an annual time scale, new production
in the open ocean is believed to be roughly one-third to one-half of regenerated production,
but the actual valué may vary considerably from these figures over short temporal and
spatial scales. P.Q., photosynthetic quotient (defined in the text).

NEW
PRODUCTION

(P.CU1.8)

REGENERATED

PRODUCTION

(P.Q.sl.2)

)

recycled nitrogen
(NH 4 + etc.)

upwelled , V

N q - sedimentation

3 of organic -N

nutricline by vertical mixing, but it also ineludes smaller amounts of nitrogen
entering by N 2 -fixation and through river inflow and precipitation. Recycled
nitrogen is primarily in the form of ammonia and urea. This comparison of
regenerated and new nitrogen (and of regenerated and new production, see
Figure 5.20) is important because only the continual input of new nitrogen
can determine the total capacity of the ocean to produce a sustainable fish
harvest (remember that removing fish from the ocean also removes nitrogen).
It is also only the new nitrogen that can help to take up the excess CO 2 that
is believed to be entering the ocean from hurtian activities; in this case,
increased production of phytoplankton removes more carbón dioxide.

In oligotrophic areas of the oceans (e.g. the large convergent gyres discussed
in Section 3.5.1), there is little upward movement of water from below the
euphotic zone and thus the amount of new nitrogen is very small. In
upwelling regions, however, the amount of new nitrogen is very large. The
ratio of new production to total (new + regenerated) production is referred
to as the/-ratio. This is shown in Figure 5.21a where it is related to waters
of different nutrient concentrations. The valué of the /-ratio is probably 0.1
or less in oligotrophic waters, but may be as high as 0.8 in upwelling zones.
An annual average for the whole ocean is estimated to be about 0.3 to 0.5.
About one-third of the global pelagic primary production takes place in areas
where new nitrogen is entering the euphotic zone; these Coastal or upwelling
areas represent only about 11% of the ocean surface (see Table 5.1).
Elsewhere, primary production depends predominantly on nitrogen that is
recycled within the euphotic zone.

140

QUESTION 5.14 (a) If the annual primary production of a marine area is

300 g C m _: and 30 c k of that production is driven by regenerated nitrogen,
what is the valué of the /-ratio? (b) Where would you expect to find water
with this amount of production and with this /-ratio? (Refer to Section 3.6.)

Figure 5.21b relates the production of carnivorous fish plus squid to total
primary production and to new production. The total amount of fish and
squid produced results from both regenerated and new production.

Harvesting that fraction of fish produced from recycled nitrogen will lead to
a loss of nitrogen from the System and thus a decrease in production, but
removing fish produced from new nitrogen sources will result in a
sustainable harvest because the nitrogen will be replenished. Thus a larger
fraction of the total fish production can be removed from eutrophic waters
than from oligotrophic waters without depleting nitrogen in the surface
layers. Figure 5.21b shows that a total fish production of 2 g wet weight
m~ 2 yr _l in oligotrophic waters with an /-ratio of about 0.1 will give a
sustainable annual harvest of 0.2 g wet weight m“ 2 yr' 1 (from 0.1 of 2 g).

In comparison, 20 g wet weight m' 2 yr -1 of total fish production in
eutrophic waters with an /-ratio of 0.8 provides a sustainable harvest of
16 g m -2 yr _l , or an 80-fold increase in the sustainable harvest compared
with only a 10-fold increase in total fish production.

QUESTION 5.15 Why does the amount of fish available for a sustainable
harvest increase exponentially in going from oligotrophic to eutrophic waters
in Figure 5.21? The answers should be apparent in Figures 5.21a and 5.21b.

In Figure 5.20, the photosynthetic quotient (PQ = moles of O 2 produced by
the phytoplankton divided by the moles of CO 2 taken up) is used to diagnose
the difference in the production processes involving the two types of
nitrogen. Note that regenerated production based on recycled nitrogen has a
lower PQ (% 1.2) than new production (PQ ~ 1.8) that is based on nitrogen
entering from the atmosphere and rivers and, especially, on upwelled nitrate.
The reason for this is summarized in Figure 5.22, where different valúes of
PQ are obtained depending on which type of nitrogen is being utilized and
which Chemical pathway is being followed in photosynthesis. If only
carbohydrate material were being formed, there would be a stoichiometric
yield of one mole of oxygen produced for each mole of carbón dioxide used.
However, lipid material also is formed during photosynthesis, and because
lipids are more highly reduced than carbohydrates, additional oxygen will be
released and the PQ will approximate 1.2 instead of 1.0. If large amounts of
nitrate (a new nitrogen source) are taken up and reduced in the process of
forming proteins, even more oxygen is liberated relative to the CO 2 utilized,
and thus the PQ will increase to about 1.8. If ammonia (a recycled nitrogen
form) is used as a nitrogen source for protein manufacture by the
phytoplankton, then oxygen is required in the process and the PQ would be
1.0 or less. Thus rapidly dividing phytoplankton populations, using nitrate,
will have relatively high PQ valúes. Lower PQ valúes indicate that reduced
States of nitrogen, like recycled ammonia, are being used in photosynthesis.
Because it is difficult to make accurate measurements of the PQ or,
altematively, of the rate and amount of vertical nitrate transport, the /-ratio
remains a matter of much discussion and debate among scientists.

141

ipd

+ 0 2 PQ* 12

COj+HjO

(-CH P 0-J

carbohy tírale

+ 0-, P.Q. s 10

N0 3 -

(NH 2 ) +0 2 P.Q = 1.8

mírate

protein

ammo

NH 4 *

oí

+ H 2 0 PQ. = 0,8

Figure 5.22 The relationship between photosynthetic pathways and the photosynthetic quotiení
(P.Q.). The basic equation for photosynthesis is shown in the middle; in this case, only
carbohydrate is being produced and the P.Q. would be 1.0 (1 mole of 0 2 is produced for each
mole of C0 2 utilized). Note that the valué of P.Q. varíes according to the type of nitrogen source
being utilized by the phytoplankton to form proteins. (See text for further details.)

5.5.2 CARBON

Carbón is another element that is essential for life, but unlike nitrogen,
carbón is never present in the sea in limiting quantities. However, the carbón
cycle (shown in Figure 5.23) has some special properties that involve both
physical and biological processes.

Carbón dioxide enters the ocean from the atmosphere because it is highly
soluble in water. If the concentration of C0 2 in seawater depended entirely
on the partial pressure of C0 2 in the atmosphere (0.3 mi l -1 ), on the relative
concentrations of C0 2 in water and air, and on the temperature and salinity
of the water, then the amount of C0 2 in seawater would be very low. In the
sea, however, free dissolved C0 2 combines with water and ionizes to form
bicarbonate and carbonate ions, as shown below.

C0 2 INPUTS
atmosphere;
respiration;
mineralization; and
dissolution of CaC0 3

C0 2 + H 2 0 H 2 C0 3

- HC0 3 + H +

carbonic

acid

bicarbonate

ions

C0 2 UTILIZATION
photosynthesis; and
formation of CaC0 3

C0 3 - + H +

carbonate

ions

142

These ions are bound forms of carbón dioxide, and they (especially
bicarbonate) represent by far the greatest proportion of dissolved carbón
dioxide in seawater. On average, there are about 45 mi total CO 2 1 _1 of
seawater, but because of the equilibrium Chemical reactions shown above,
nearly all of this occurs as bound bicarbonate and carbonate ions which thus
act as a reservoir of free CO 2 . The amount of dissolved CO 2 occurring as
gas in seawater is about 0.23 mi l -1 . When free CO 2 is removed by
photosynthesis, the reaction shifts to the left and the bound ionic forms
release more free CO 2 ; so even when there is a lot of photosynthesis, carbón
dioxide is never a limiting factor to plant production. Conversely, when CO 2
is released by the respiration of plants, bacteria and animáis, more
bicarbonate and carbonate ions are produced.

Note that hydrogen ions are liberated in the general Chemical reactions
shown above. This means that the pH of seawater is largely regulated by the
concentrations of bicarbonate and carbonate, and the pH is usually 8 ± 0.5.
When CO 2 is added to seawater due to mineralization processes and
respiration, the number of hydrogen ions increases and the pH goes down
(the solution becomes more acidic). If CO 2 is removed from the water by
photosynthesis, the reverse happens and the pH is elevated. Thus seawater
acts as a buffered solution.

Some marine organisms combine calcium with carbonate ions in the process
of calciflcation to manufacture calcareous skeletal material. The calcium
carbonate (CaCC> 3 ) may either be in the form of calcite or aragonite, the
latter being a more soluble form. After death, this skeletal material sinks and
is either dissolved, in which case CO 2 is again released into the water, or it
becomes buried in sediments, in which case the bound C0 2 is removed from
the carbón cycle.

QUESTION 5.18 Which marine organisms (planktonie, nektonie and benthic
speeies) incorpórate carbón dioxide into carbonate skelelons and thus
influence the carbón cycle? íRefer to Sections 3.1 and 4.2 for pan of the
answer.)

The simplified carbón cycle shown in Figure 5.23 summarizes the processes
discussed above. In general, CO 2 is converted from inorganic to organic
carbón by the photosynthesis of the phytoplankton. This is then consumed by
the higher trophic levels, and some CO 2 is recycled as inorganic bicarbonate
while some may be lost from the ocean surface in gaseous form. Carbón
dioxide is absorbed at the ocean surface and is produced in the water column
by respiration and mineralization processes. It is believed that more carbón
dioxide is being absorbed by the oceans than is being lost to the atmosphere.

The total amount of soluble carbón dioxide (bicarbonate and carbonate ions
plus dissolved CO 2 ) in the world’s oceans is estimated to be about 38 x 10 12
tonnes. This is about fifty times more than the total carbón dioxide in the
atmosphere. The burning of fossil fuels is increasing the total carbón dioxide
in the atmosphere at a rate of about 0.2% per year, and it is very important
to know if this increase can be absorbed by the oceans, or if it will continué
to accumulate in the atmosphere where it may contribute to global warming
by the process often referred to as the ‘greenhouse effect’.

The importance of the ocean’s biology to the carbón cycle and to the
balance of CO 2 in the atmosphere is threefold. Firstly, the amount of CO 2

143

atmospheric

* cg 3 -

voléame

emissipn

free dissoived
/ CO,

bicarbo nale
and

carbonate

free dissolved

' CCW

respiralion

respiration

benihos

- ;
ptiotDsyMhesis

zooplankíon

and

rekíon

respiration

d resolved
/and
particülale
^ detritus

decomposition

bacteria

s CaCO, „
skeletons

sinfcing

deep-sea

sediments

Processes

Livjng material

Norhlfving material

Figure 5.23 The basic scheme of the carbón fixed by the food chain depends on how much new nitrate enters the
cyde- euphotic zone to support photosynthesis (see Section 5.5.1). Secondly, the

amount of carbón which can be permanently lost to the sediments depends
on deep-water chemistry, ecology and sedimentation processes, and
particularly on the bacterial loop (Section 5.2.1) which recycles dissolved
and particulate organic carbón. Thirdly, the amount of carbón dioxide taken
up in the carbonate skeletons of marine organisms has been, over geological
time, the largest mechanism for absorbing CO 2 . At present, it is estimated
that about 50 x 10 15 tonnes of CO 2 occurs as limestone, 12 x 10 15 tonnes in
organic sediments, and 38 x 10 12 tonnes as dissolved inorganic carbonate.
Determining the amount of carbón that is transferred along the various
pathways in Figure 5.23 is a difficult problem in both geochemistry and
biology, but a necessary exercise if we are to solve the global carbón budget.

CUESTION 5.17 Carbón dioxide is essential to the process of photosynthesis.
Should it be considered as a nutriera, like nitrate for example. that can limit
the rate of phytoplankton production in the oceans?

5.6 SliMMARY OF CHAPTER 5

1 Food chains are ways of describing the linear passage of energy and
organic materials contained in food from the first trophic level of primary
producers, through the consumer levels of herbivores and camivores, to the
top-level predators. There is an energy loss with each transfer between
trophic levels because of metabolic demands and conversión of Chemical
energy to heat. However, Chemical elements that are incorporated in food are

144

recycled through the decomposition of organic materials; this process
releases dissolved inorganic compounds that can once again be taken up by
phytoplankton and converted to organic compounds during photosynthesis.

2 Despite great differences in size between phytoplankton and consumers
in higher trophic levels, the differences in generation times (hours to many
years) among these organisms result in very similar biomass valúes in each
trophic level of marine food chains.

3 Estimates of secondary production in different marine localities can be
made by using the expression 1 ) = P\E n . This equation combines
quantitative valúes for primary productivity in an area with the number of
trophic levels in the food chain, and with the ecological efficiency at which
energy is transferred from one trophic level to another.

4 The number of trophic levels in a food chain is inversely correlated with
the predominant size of the phytoplankton. Food chains in nutrient-rich
upwelling areas are characterized by having large chain-forming diatoms,
high primary productivity, few trophic levels, and a high biomass of fish or
marine mammals. In the nutrient-poor open ocean, the primary producers are
nanoplanktonic autotrophic flagellates with relatively low productivity; this
leads to long food chains and, because of increased energy loss in longer
food chains, there is a relatively lower biomass of top-level predators.

5 Food webs are more realistic, but more complex, depictions of energy
flow through interacting species. They are a means of recognizing that many
marine species compete for the same food Ítems, that many animáis change
diets during life, that some organisms feed primarily on detritus, and that
cannibalism is common in the sea. Such relationships may affect the amount
of energy that is available for top-level predators; for example, competition
for food between ctenophores and larval fish, or ctenophore predation on fish
eggs and fish larvae, may significantly lower fish stocks that are harvested
commercially.

6 Bacteria and planktonic protozoans internet in a microbial loop that is
coupled with the elassie phytoplankton-zooplankton-fish food chain. In this
subsystem, bacteria decompose particulate and dissolved detritus; the
resulting bacterial production is consumed by protozoans and by some larger
zooplankton, such as invertebrate larvae and appendicularians. Thus bacteria
regenérate dissolved nutrients for subsequent utilization by phytoplankton,
they themselves form a source of food for planktonic bactivorous species,
and the bacterial production is transferred to higher trophic levels by the
intermediary links of protozoans which are fed on by larger plankton.

7 Major differences exist between marine and terrestrial food webs. The
majority of marine primary production is carried out by fast-growing
microscopic phytoplankton, most of which is consumed and assimilated by
herbivores. In contrast, most terrestrial vegetation is large, slower growing,
and contains much indigestible structural material. Only 15% or less of the
total terrestrial plant production is eaten, and only a fraction of this is
digestible and assimilated into herbivore production. Further, the dominant
marine animáis are poikilothermic, with lower metabolic energy demands
than the homoiothermic birds and mammals that live in terrestrial habitats.
This difference in energy utilization, coupled with the fact that most marine
primary production is eaten, means that energy is transferred with greater
efficiency through marine food chains, and that there is a much higher
secondary production in the sea compared with that on land.

145

8 Whereas it is possible to obtain fairly accurate estimates of primary
production through various techniques, and ñsh catch statistics provide
mínimum valúes for energy output from marine food webs, it is much more
difficult to quantify secondary production in the intermedíate trophic levels
occupied by zooplankton and smaller nekton. Although some techniques
have been applied to measure secondary production from field data, these are
often impracticable because of the vast geographic areas under consideration
and the continual movement of the water and resident organisms. Many
researchers have therefore resorted to different experimental options. These
inelude laboratory-scale experiments, which attempt to quantify each aspect
of energy partítioning in a species; the use of controlled ecosystem
experiments, which are carried out on a larger scale and attempt to study
several interacting trophic levels at one time; and Computer model
simulations, in which data from various sources are entered into
mathematical models that attempt to simúlate natural processes.

9 The term ‘mineralization’ describes the process whereby elements that
have passed through food webs are reeyeled. Ecological studies are
particularly concerned with the reeyeling rates of essential nutrients that may
be present in limiting concentrations in the sea; these inelude nitrate, iron,
phosphate and, occasionally, dissolved Silicon. Of these nutrients, nitrate is
the one that is most often present in sufficiently low concentrations to limit
plant growth. Nitrogen has a complex eyele in the sea because it occurs in
many forms; nitrate is the dominant form most often utilized by
phytoplankton, but ammonia, nitrite, and dissolved molecular nitrogen can
also be used by some species. The physiological activities of organisms
produce particulate and dissolved organic nitrogen in various Chemical
species, and different types of bacteria medíate the conversions from one
type of nitrogen compound to another.

10 An important distinction is made between regenerated nitrogen
(primarily ammonia and urea) that is reeyeled in the euphotic zone by
pelagic organisms, and new nitrogen (primarily nitrate) that enters the
euphotic zone from upward movement of deep water or, in smaller amounts,
from river inflow and precipitation. The amount of new nitrogen relative to
regenerated nitrogen (the /-ratio) is high in upwelling regions and low in
oligotrophic areas. Where production is based primarily on new nitrogen
(nitrate), the photosynthetic quotient (PQ) is high, indicating the formation of
proteins from nitrate with release of oxygen. If photosynthetic production is
based on regenerated nitrogen forms (i.e. ammonia), the PQ is low as oxygen
is required in the reaction. It is the continual input of new nitrogen that can
elevate primary production levels and ultimately sustainable fish harvests.

11 Carbón is essential for Ufe, and is never present in limiting quantities in
the sea. This is because dissolved carbón dioxide enters into equilibrium
reactions with bicarbonate and carbonate ions. As more CO 2 enters the sea
from the atmosphere, or as the result of physiological activities (primarily
respiration), more bicarbonate and carbonate ions are formed, thus increasing
the amount of CO 2 which can continué to enter the sea. Conversely, when
there is a biological demand for dissolved CO 2 , the Chemical reactions are
reversed and CO 2 is released from its bound ionic States. As the quantities of
CO 2 entering the atmosphere from human activities increase, it becomes
increasingly important to determine how much can be absorbed by the seas
and how much will accumulate in the atmosphere where it may contribute to
global warming.

Noxv try the following questions to consolídate your understanding ofthis
Chapter.

QUESTIOH 5.18 Assume that in an open ocean región the priman production
is 1000 g wet weight m 2 yr _l , and there is a 20% transfer efficiencv
beiween the primary producers and primary consumera, and 10% efííciency
between all successive trophic levels. What is ihe máximum amount of the
priman' production that can be eonverted to íish in the highest trophic level?

QUESTI0N 5*19 Considering the relatíve age when different types of
phytoplankton and different top predators ürst appeared in the geologic
record (see Appendíx I), what can you deduce about the evolution of the
types of food chains shown in Figure 5,3?

0UESTI0N 5.20 What might happen to a regional ítshery if mosi of the
piscivorous Iish (e.g* tuna) were removed, leavíng mosUy planktivorous íish
(e.g. sardines) to harvest? Consider the effects on the regional food Chain in
terms of energy transfer and relatíve numbers of organisms in different
trophic levels,

QUESTI0N 5,21 In one región of the North Atlantic, the production of sand
eels, which feed on zooplankton, has beert determined from Iish catch data to
be 0*5 tonnes wet weight per hectare per year, and the phytoplankton
production in the same area is 200 g C m : yr E * Assuming that carbón
makes up 50% of the drv weight of Iish and that the dry weight is 20% of
the wet weight. what is the uverage ecological efííciency oí this system?
(Note: 1 hectare — 10000 nr.)

QUESTIOH 5*22 In each of the following situatiom, would it be best to use a
laboratory experiment. un enclosed experimental ecosvstem, or a Computer
model simulation.?

(a) to study the effeet of pesticide runoff from agricultura! land on neritic
walers:

(b) lo examíne the potential environmental impaets of Hamming an estuary:

QUESTIOH 5,23 The chaetognath Sagina elegtms consumes 5 mg of copepods
per day and produces 0*75 mg of faecal material per day. What is the
assimilatkin efficiency oí this camivore?

QUESTIOH 5.24 Would it be praeüeal to add nutrients to a snía II hay in order
to enhance oyster cultures?

QUESTIOH 5*25 Assuming that the amount of new nitrogen remains constant.
what would happen to the productivity of an ocean area if many of the
residem Iish were removed by a commercial fishery?

CHAPTER

NEKTON AND FISHERIES OCEANOGRAPHY

Fish make up the largest fraction of the nekton, but large crustaceans, squid
and related cephalopods, sea snakes, marine turtles, and marine mammals
can be important nektonic species in certain areas. Large nektonic animáis
and seabirds can have profound influences on marine communities in terms
of predation. As well, many of these animáis figure importantly in
commercial harvests as sources of food, fur, or other commodities, or they
have done so in the past. Fish dominate the present marine catch, and squid
are being taken in increasing numbers; the catch of marine mammals and
marine turtles is declining through public pressure for conservation of many
of their species.

6.1 NEKTONIC CRUSTACEA

Although there are pelagic swimming crabs and shrimp that fall in the
category of nekton, relatively little is known about the biology of the
different species and few of them are abundant enough to be of commercial
interest. Ninety-five per cent of commercially harvested crustaceans are
demersal species that are caught in benthic trawls. However, considerable
attention has been given to euphausiids (see Section 4.2) as an exploitable
resource, as one species in particular is exceedingly abundant.

With the decline in whale numbers and the consequent cessation of most
commercial whaling, Euphausia superba , the Antarctic krill and predominant
food of baleen whales (see Figure 5.4), became an alternad ve fisheries target.
Although rarely used for human food, these large (5-6 cm long) euphausiids
can be dried and processed into feed for livestock, poultry, and farmed fish.
Russian and Japanese fieets began harvesting krill in the 1960s, and the peak
catch was 446000 tonnes in 1986. Economic considerations closed the
Russian harvest, and in 1994 the commercial harvest was only about 100000
tonnes, taken by fieets from Japan and Chile. This is an insignificant fraction
of the amount consumed by natural predators, estimated at about 470 million
tonnes annually (see Table 5.2). The potential krill harvest has been
estimated to be at least 25-30 million tonnes a year, or about one-third of
the present world fish catch. However, the economic costs of fishing in the
remóte Antarctic are relatively high and, although krill form vast swarms,
the congregations are widely scattered and sometimes located at depths of
150-200 m. Nevertheless, once a swarm has been located by echo-sounding,
a single net haul from a large fishing vessel may commonly catch 10 tonnes
of krill. The ecological consequences of removing vast numbers of krill on
the balance of populations in the Antarctic ecosystem (including recovery of
whale numbers) are not clear but, because krill are central to the Antarctic
food web, there is reason for caudon in expanding this harvest.

There is also a commercial harvest of a smaller euphausiid, Euphausia
pacifica , along the northeastern Japanese coast. This particular fishery
depends upon the unique fact that, in this area during the spring, the
euphausiids form surface schools and are therefore easily accessible. The
harvest of about 60000 tonnes per year is mainly processed for use as feed

148

in fish farms. The euphausiids provide a rich source of both protein and
vitamin A; the latter is believed to enhance the texture and pigment of the
flesh of the farmed fish.

6,2 NEKTONIC CEPHALOPODS

Squid (Figure 6.5g), cuttlefish, and octopods (e.g. Octopus) are the
molluscan members that make up the Class Cephalopoda. Squid constitute
approximately 70% of the present catch of cephalopods, and estimates
indicate that the harvest could be increased appreciably. The potential world
catch of squid is estimated conservatively to be 10 million tonnes annually.
Despite their abundance, surprisingly little is known of the biology and
ecology of many species.

Squid range in size from a few centimetres up to the legendary deep-sea
giant squid {Architeuthis) that exceeds 20 m in length (with outstretched
tentacles) and 270 kg in weight, and thus attains the status of being the
largest of all living invertebrates. All squid swim by propulsión, ejecting jets
of water from their siphon, and these streamlined cephalopods rival fish in
swimming ability and manoeuverability. Some of the larger squid species are
capable of speeds of about lOms" 1 . They are also rivals with some fish for
food, as squid typically eat 15-20% of their body weight per day, taking a
variety of zooplankton as well as smaller fish and other squid as prey.

Many of the very abundant squid species are extensively fished and form a
major source of human food in some countries. In 1981, the Japanese began
using driftnet fishing to harvest squid in the Pacific Ocean. Driftnets are
panels of monofilament webbing measuring 8-10 m in width and up to
50 km in length; mesh size is usually 90-120 mm. At night, the nets were
placed vertically in the open ocean and allowed to drift with the winds and
currents for about 8 hours in order to snare squid and fish. By 1989, Japan,
Korea, and Taiwan were deploying about 800 driftnet vessels in the Pacific
to harvest 300000 tonnes of squid annually, and an estimated 200 other
vessels were operating in the Atlantic and Indian oceans. In addition to
squid, these almost invisible nets indiscriminately captured a large number of
other species. In the North Pacific, salmón were the most common (and
illegal) by-catch; in the South Pacific, albacore tuna were a valuable catch
totalling 60000 tonnes in 1988. By 1989, there was growing concern about
the numbers of animáis being captured incidentally by this method. In
addition to nonselective capture of other fish species (including sharks) and
turtles, it was estimated that between 750 000 and 1 000000 seabirds and
20000 to 40 000 marine mammals were being killed annually in the nets
used in the Pacific alone. There were no estimates of the numbers of large
zooplankton, such as salp chains and jellyfish, that may also have been
destroyed by the nets. The seriousness of removing such vast numbers of
animáis from the oceans led the United Nations General Assembly to accept
a resolution calling for an intemational moratorium on all large-scale, high
seas, driftnet fisheries in 1993. Although some driftnets continué to be used,
the problem has abated.

QUESTIQN 6.1 Asstiming a nightly deploymeni of 800 driftnets during the
I980s, how many küometres of net were set out eaeh night by the squid
fleets in the Pacific Ocean?

149

Squid can be harvested by more selective fishing methods. In Japan, about
500 000 tonnes of Todarodes pacificus are caught each year using a
technique that captures only squid. This particular squid undertakes an
extensive annual migration of about 4000 km, moving from the spawning
grounds in the northern part of the East China Sea (about 32°N) to the
vicinity of the Kurile Islands (45°N) before returning.

6.4 MARINE MAMMALS

There are comparatively few reptiles that have adapted to a marine life. The
best known are the eight species of marine turtles, but there are more than
six times as many species of sea snakes, and there is one marine lizard, a
large seaweed-eating iguana of the Galápagos Islands. Several crocodiles live
in Coastal waters, the largest being the infamous Australian species
Crocodylus porosus.

Marine turtles usually are found in tropical waters, but some migrate or are
carried by currents to températe shores. Some turtles feed on jellyfish or fish
in the open ocean, others (the green turtle) on shallow-water seagrasses, but
all undertake long migrations to return to land in order to lay their eggs at
specific nesting sites on sandy shores. High mortality is infíicted on the eggs,
which are eaten by natural predators and are also prized by humans. Newly
hatched young also have high mortality rates as they are preyed upon by
birds and crabs during their scramble for the sea, and by predatory fish
during their early life in the water. Adults have been hunted nearly to
extinction for their meat and decorative shells. All sea turtles are now
considered to be threatened or endangered species and conservation methods
are in forcé in many countries, including bans on capture and importation of
turtle producís. Efforts are being made in several areas of the Indo-Pacific
and Caribbean to gather eggs and keep these until hatching, at which time
the young are released directly into the sea. It remains to be seen whether
these protective measures will restore population numbers.

Sea snakes breathe air by means of nostrils and lungs, but they are truly
marine animáis that inhabit Coastal estuaries, coral reefs, or open tropical
water. Most of the approximately 60 species remain at sea to bear their live
young. They school in large numbers and feed on small fish or squid which
they kill with venom injected by fangs. Sea snakes are extremely poisonous
and, although not all are aggressive, they have caused human deaths. They
themselves have few predators except for sea eagles, sharks, and saltwater
crocodiles. Sea snakes are presently restricted to warm waters of the Indian
and Pacific oceans, but there has been concern that a new sea-level canal
through the Isthmus of Panama would allow their passage into the warm
waters of the Caribbean and Atlantic. They are presently excluded by a
freshwater barrier in the canal.

6.3 MARINE REPTILES

There are three orders of mammals that have evolved from different
terrestrial ancestors and independently adapted to life in the sea. These three
orders inelude respectively: the whales, dolphins and porpoises: the seáis,
sea lions, and walruses; and the dugongs, manatees, and sea cows. All share

150

the mammalian characteristics of being warm-blooded (homoiothermic) and
nursing their young, and they all rely on breathing air.

The order Cetácea comprises the 76 or so species of marine mammals
known as whales, porpoises, and dolphins. The ancestors of this group were
land animáis that entered the sea about 55 million years ago. The largest of
these marine mammals are the baleen whales (Figure 6.1); these inelude the
biggest animáis that have ever lived, the blue whales, which can attain a
length of 31 m.

Baleen whales form a sepárate suborder (Mysticeti) of about ten species.
Like the largest of the sharks, most of these immense whales feed primarily
on zooplankton that they strain through specialized homy plates called
baleen or whalebone. The brush-like baleen hangs down from the roof of
the mouth on both sides, and food that collects on the baleen is periodically
removed by the tongue of the whale. The humpback and finback whales also

J_i 1 i_I_i- - i

Figure 6.1 Relative sizes of baleen and

15 18

toothed whales.

metres

151

are capable of capturing schools of relatively large fish, such as mackerel
and herring, and the grey whale suction-feeds on benthic animáis.

Some of the large baleen whales (e.g. greys, humpbacks) make extensive
seasonal migrations, usually breeding in winter in tropical waters and
moving poleward to feed in summer. Smaller cetaceans do not undertake
long migrations, but move in response to changing food supplies or physical
changes.

QUESTIQN 0.2 Oí what advamage is it to mígrating whales to have
warm-water breeding nursery areas and high-latitude summer feed i ng sites?

The suborder Odonticeti ineludes the other 66 species of cetaceans, all of
them equipped with teeth and characterized by having a single blowhole
instead of the two of baleen whales. The odontocetes inelude the remaining
whales, dolphins, and porpoises (Figure 6.1). The toothed whales are
formidable predators in the sea, taking squid or fish as prey or, in the case of
some killer whales, even other whales or seáis and sea lions. Unlike some of
the baleen whales, these animáis are not reliant on surface-living prey, and
they may undertake dives to depths of several hundred metres. The sperm
whale holds the record among marine mammals for deepest dives; it is
believed to descend to over 2200 m in search of giant squid. Some
odontocetes hunt prey by echolocation, in which they emit pulses of sound
and monitor the returning echoes, and at least some species show
cooperative behaviour in herding and capturing their prey.

Some scientists have suggested that the cetaceans as a whole, and possibly
the sperm whales alone, consume a greater quantity of prey than is taken by
the entire world commercial fishery. For example, between 1979 and 1982,

18 species of cetaceans consumed between 46000 and 460000 tonnes of
prey annually on Georges Bank, off the northeastem United States,
compared with a commercial harvest in the same area of 112000-250000
tonnes. In the Mediterranean, where squid figure importantly in the human
diet, cetaceans are estimated to eat about 2.3 times more squid than are
taken by humans. And before commercial whaling fleets decimated whale
populations in the Antarctic, baleen whales may have taken about 190 x 10 6
tonnes of krill annually, a figure that represents more than twice the total
world catch of all marine species (see Section 6.7.1). Given these figures, it
is not surprising that fishermen often regard cetaceans as competitors for fish
and squid stocks.

The Inuit have been hunting marine mammals since time immemorial, but
the earliest recorded whaling began off northem Europe between A.D. 800
and 1000 and whaling became a major commercial enterprise in the 1700s
and 1800s. Whales were exploited primarily for their oil, which was used in
lamps, and for whalebone (baleen) used to stiffen women’s apparel; whale
meat was of secondary importance, except in Japan. The advent of increased
mechanization, motorized high-speed ships, and explosive harpoons in the
early 1900s resulted in rapid declines in whale populations and threatened
extinction of some species. Even after the establishment of the International
Whaling Commission (IWC) in 1946, the annual catches of whales
continued to climb to about 65 000 during the 1960s. Pre-exploitation and
present population estimates of 13 species are given in Table 6.1; of these
species, nine have been listed as endangered or vulnerable species since
1970. Given the low fecundity and slow development times of the great

152

Table 6.1 Past and present population estimates of whales. All estimates are taken
from the International Whaling Commission and most are highly speculative.

Common ñame

Scientific ñame

Population estimates

Pre-exploitation

Present

Baleen whales:
Blue

Balaenoptera musculus

228000

<10 000*

Fin

Balaenoptera physalus

548000

150000*

Sei

Balaenoptera borealis

256000

54000*

Bryde’s

Balaenoptera edeni

100000

90000

Minke

Balaenoptera acutorostrata

140000

725 000

Bowhead

Balaena mysticetus

30000

7800*

Northern right

Eubalaena glacialis

No estimate

<1000*

Southern right

Eubalaena australis

100000

3000*

Humpback

Megaptera novaeangliae

115 000

10000*

Grey

Eschrichtius robustas

>20000

21000*

Toothed whales:
Sperm

Physeter catodon

2 400000

1 950000*

Narwhal

Monodon monoceros

No estimate

35 000

Beluga

Delphinapterus leucas

No estimate

50 000

*Listed as an endangered or vulnerable species by the International Union for the
Conservation of Nature or by the United States Government.

whales, severely depleted populations may take decades to recover. Only one
species, the minke whale, is known to have greatly increased in abundance.
The relatively small minkes were never heavily exploited, and the Southern
population might have increased due to the decline of larger baleen species
with which it may compete for krill. In 1986, the IWC voted to establish an
indefinite ban on commercial whaling in the hope of re-establishing
endangered stocks. In 1994, the IWC designated 28 x 10 6 km 2 around
Antárctica as a whale sanctuary, thus providing permanent protection for
about 90% of the world’s whales. These measures do not, however, slow
cetacean mortality due to other factors. The smaller dolphins and porpoises
are captured incidentally by fishing gear; and cetaceans that reside in, or
temporarily enter, Coastal waters are subject to increasing habitat destruction
and pollution. There is no doubt that pollution threatens beluga whale
populations in the St. Lawrence Estuary of eastem Cañada, and river
dolphins in many other areas.

A second order of marine mammals ineludes the seáis, sea lions, and
walruses. These familiar animáis are known taxonomically as pinnipeds
(order Pinnipedia), meaning Teather-footed’ to describe their four swimming
flippers. In contrast to whales, these animáis spend part of their time on land
or on ice floes, where they congrégate for breeding and resting. The 32
species of pinnipeds are found in all the seas of the world, and there is one
freshwater species in Lake Baikal, but the majority of species and the largest
populations are found in the coid waters of the Arctic and Antarctic. Most
feed primarily on fish or squid, but walruses also use their tusks to dig
molluscs and other benthic animáis from the sea bottom. Pinnipeds typically
live and travel in herds, and some may undertake long migrations at sea.

Although seáis and sea lions have been heavily exploited in the past for their
fur and oil, and walruses for their ivory, hunting pressure has lessened for
most of the species. However, the Caribbean monk seal is now believed
extinct, and the Hawaiian and Mediterranean monk seáis remain endangered.

153

Manatees and dugongs belong to the mammalian order Sirenia. They are the
only herbivorous aquatic mammals, and they rely on larger plants, not algae,
for nourishment. Their food requirements restrict them to living in shallow
Coastal waters, estuaries, and rivers. All four species of this order reside in
warm waters and do not come on to land. Manatees and dugongs are thought
to have been highly social animáis, as oíd records report huge congregations
of these animáis before hunting decimated their numbers. The few remaining
individuáis have tended to become solitary or form only small family
groups. The sirenians have been particularly vulnerable to hunting pressure
because of their inshore habitats and their slow and placid behaviour, and
they are prized for their meat, oil and hides in many cultures. At one time,
dugongs had a widespread distribution which included Atlantic waters;
today, they are restricted to the Indian and Pacific oceans. All three species
of manatees are found only in tropical Atlantic waters.

A fifth species of sirenian, Steller’s sea cow (Hydrodamalis gigas), became
extinct within historical times. The existence of these huge animáis was
documented only once, by Georg Wilhelm Steller who was acting as a
physician and naturalist on an expedition sailing in the far North Pacific
under Commodore Vitus Bering. The St. Peter was shipwrecked in 1741 on
a small island near the western end of the Aleudan chain. Although Captain
Bering and many of his crew died from sickness in the first days after
landing, the remaining survivors found adequate food in the resident otters
and seáis. They also discovered sea cows in nearby waters and were
eventually able to capture these for a source of meat and oil. The animáis
were described as attaining a length of up to 10 m and a weight of 10 tons.
They fed on kelp and other large seaweeds, chewing the plant blades with
horny plates covering the palate and jaws. They formed small herds, and
were slow-moving and of a passive temperament. The shipwreck survivors
brought word of their discoveries when they returned to Russia in 1742, and
future whaling and fur-hunting expeditions to the Bering Sea began to rely
on sea cows as a source of food during the winter. Although 2000 Steller sea
cows were estimated to live in the región, the last was killed in 1768, only
27 years after the discovery of this population. Fossil evidence indicates that,
as recently as 20 000 years ago, this sea cow inhabited Coastal areas as far
south as California; although the cause of its extinction in these areas is a
matter of conjecture, the animal was probably easy prey for pre-historic as
well as modern humans.

6.5 SEABIRDS

Like the marine reptiles and mammals, seabirds have evolved from land
species that readapted to life in the sea. There are now approximately
260-285 species of seabirds, depending on how the term is defined; these
species represent roughly 3% of the world’s birds. Those birds that are most
highly adapted to the marine environment inelude the auks, albatrosses,
petrels, penguins, and gannets, all of which have few representatives on land
or freshwater and spend 50-90% of their lives at sea. At the other extreme
are the shorebirds, like sandpipers and plovers, that depend upon marine
food sources but are incapable of swimming.

The many species of oceanic birds have developed diverse methods of
feeding and take different types of prey (Figure 6.2); this is reflected in
species differences in structure of the bilí and wings, Some species

storm petrel prion

petrel

giant

tulmar

albatross

Surface

seizing

r* phalarope

Figure 6.2 Seabird feeding methods. In most cases, the examples given represent only one
of several types of birds that feed in the depicted manner.

(skimmers, gulls, petrels) skim neuston from the immediate surface layers of
the ocean, others (pelicans, tems, gannets) plunge deeper into the water to
seize zooplankton, squid, or fish. Penguins, cormorants, murres, and puffins
actively pursue their prey underwater, using their wings or feet for
swimming. Although emperor penguins may dive to depths of more than
250 m, the majority of seabirds are essentially dependent on the uppermost
layers of the sea for their food. The impact of seabird predation on oceanic
surface life has often been neglected, but it may be considerable.

Although seabirds are found world-wide, the largest colonies are located
adjacent to highly productive ocean areas where food is plentiful and
concentrated. Millions of penguins (six species) are present in the Antarctic,
where they depend for food on the vast numbers of krill or the abundant fish
and squid that occur in this productive ocean. Equally large numbers of birds
form island colonies in the upwelling Coastal regions off western South
America (see Figure 6.10). At sea, birds frequently form feeding
aggregations along oceanic fronts which, like upwelling regions, have
relatively high biological productivity (see Section 3.5). Far fewer birds are
present in low-productivity tropical regions. Seasonal changes in the marine
environment can be reflected in the distribution of birds, and some species
undertake long annual migrations in response to seasonal food availability
and suitable weather for breeding.

155

Shorebirds known as red knots undertake one of the longest migrations that
is linked to exploiting seasonally available marine food resources. The
American subspecies (Calidñs canutas rufa ) spends the austral summer at
the Southern tip of Argentina, feeding primarily on young mussels in
intertidal areas. It migrates northward in March, stopping to feed on clams,
mussels, and worms along the South American coast. By late May, over
100000 birds flock in Delaware Bay, along the eastem seaboard of the
United States. Their arrival is timed to coincide with the breeding cycle of
horseshoe crabs (Figure 7.7g) that come ashore in thousands to lay millions
of eggs. Their eggs provide a high energy food source for the birds, and this
fuels their remaining flight to islands in the Canadian Arctic. Each female
lays four eggs shortly after arrival at the breeding grounds; these weigh
about 75 g, or more than 50% of the female’s weight. The period spent in
the Arctic coincides with peak abundances of insect and aquatic life, and the
return southward migration in July and August occurs when local marine
invertebrate populations are highest along the Atlantic coast of America. The
birds increase their weight by about 40% within the few weeks spent feeding
intertidally, then resume their migration to South America. (See Section 8.5
for additional information on the impacts of shorebirds feeding on benthic
animáis.)

Seabirds exhibit natural fluctuations in population densities which can be
caused by climate change and subsequent fluctuations in prey availability.
This has been documented, for example, in the bird colonies found on rocky
uninhabited islands off the coast of Perú (see Section 6.7.2). Over
evolutionary time, birds evolve adaptations that permit them to exist within a
range of natural climatic variability. Unfortunately, seabirds are increasingly
faced with new sources of mortality inflicted by human activities, and these
occur with a tempo that preeludes slow evolutionary adaptation.

All seabirds depend on nesting sites on land for breeding, and it is here that
they encounter their greatest risks. Seabirds on land are exceptionally
vulnerable to predation as it is difficult for them to defend themselves, and
their eggs and young, against land mammals and snakes. Many species nest
on inaccessible rock islands that are naturally free of mammalian predators.
However, there are numerous examples of the delibérate or accidental
introduction of predators, such as cats, rats, and pigs, that have resulted in
disturbance or destruction of bird colonies.

CUESTION 6.3 What are the predators of Antarctic species of penguins?

(Reler to Figure 5.4.)

Some seabirds have been exploited for their feathers, meat, eggs, or body
oil. Only one, however, has been exterminated within historical time. This
was the great auk (Pinguinus impennis) (Figure 6.3), which lived only on
isolated islands in the North Atlantic. This Northern Hemisphere bird was
the ecological equivalent of the Southern penguins; like penguins, it was
large (to 1 m tall), flightless, and dived to pursue its marine prey. The giant
auk was discovered in 1534, when there were several hundred thousand
birds, but it was hunted to extinction within 300 years. At first hunted for
meat by local fishermen, these seabirds were later harvested commercially
for their feathers and oil. The last great auk was killed on Funk Island on
June 3, 1844. Thousands of giant auk carcasses discarded on the rocky
island provided fertilizer to nourish grasses, and today the island is a
sanctuary for puffins and murres.

156

Figure 6.3 The giant auk as painted by John James Audubon in about 1835, nine years
before this flightless bird was hunted to extinction.

Increasingly, seabirds are encountering greater mortality from Coastal
pollution. In all oil spills, seabirds are usually the most obvious victims, and
the effects of oil pollution are too well known to document here. Although
most spills are localized, the mortality may be high; for example, an
estimated 500000 birds perished from the effects of the oil spill of the
Exxon Valdez in Alaska. Pesticide residues, working their way up the food
web and accumulating in the bodies of seabirds, have caused thinning of egg
shells and reduced hatching success in pelicans, ospreys, and other species.
Other toxic Chemical pollutants which enter the sea and may affect birds
inelude organochlorines, PCBs (polychlorinated biphenyls), and heavy
metáis like mercury. Also of concern is the continuing loss of feeding and
reproductive habitats through development of coastlines (see also Chapter 9).

Increased fishing efforts have also affected seabird numbers. Large numbers
of birds have been incidentally captured and drowned in driftnets in the
North Atlantic and Pacific (see Section 6.2). Perhaps larger numbers are
affected by reduction in food, through harvests of their prey. In Norway, for
example, puffins have declined because of overfishing of the immature
herring that are their main food.

As is the case with many other animáis, seabirds are faced with new sources
of mortality and an accelerated pace of change. Their past evolutionary
experience, developed over 60 million years, will be of little valué in
establishing defences against oil spills and net-capture. This is particularly
true for those seabirds that have low fecundity and require a long period
before attaining breeding age. For some species, continued survival may
depend on legislated and enforced conservation and on protection of fish
stocks and nesting and breeding habitats.

6.6 MARINE FISH

Fish constitute the largest and most diverse group of marine vertebrates.
They are taxonomically separated into the following three classes:

157

Class Agnatha. This class encompasses the most primitive of living fish,
the jawless lampreys and hagfish. This group evolved about 550 million
years ago in the Cambrian, but presently has only about 50 species.

Class Chondrichthyes. The sharks, skates, and rays which belong to this
class are also known as elasmobranch fish; they are characterized by having
a cartilaginous skeleton and lacking scales. This is also an ancient group,
first appearing about 450 million years ago, and there are presently about
300 species.

Class Osteichthyes. This class ineludes the teleost fish which have a
bony skeleton. This is the most successful group and comprises the vast
majority of living fish, with somewhat over 20000 marine species. The
teleosts evolved about 300 million years ago.

Agnatha

The bodies of hagfish (Figure 6.4a) and lampreys are elongate and eel-like,
and the animáis lack scales. The mouth is surrounded by a sucking disk, and
most of the species are predators on other fish. The scavenging hagfish
burrow into the bodies of dead or dying prey to feed on internal parts;
lampreys are parasites which attach themselves to fish by their sucker-like
disk and cut into the flesh to feed on soft parts and body fluids. All hagfish
species are marine, whereas the different species of lampreys may be either
marine or freshwater. Even the marine species of lampreys spend part of
their lives in freshwater; the young live in rivers where they feed on small
invertebrates and possibly fish fry, and after metamorphosing, they move to
the sea to complete their development.

Chondrichthyes

Sharks (Figure 6.4c) are typically thought of as fast-swimming voracious
predators that consume large prey, but many also act as scavengers in the
sea. Paradoxically, the largest members of this group are docile feeders on
plankton; these inelude the basking shark (Cetorhinus maximus) and whale
shark (.Rhincodon typhus) which attain lengths of 14 m and 20 m,
respectively. Both species have small teeth and strain plankton from water
using specially modified gills. Skates and rays have flattened bodies, and

Figure 6.4 Primitive fishes belonging to the
Class Agnatha (a) and Class Chondrichthyes (b
and c). (a) hagfish; (b) manta ray; (c) shark.

158

most are adapted to a bottom-dwelling habitat. The majority are predators on
benthic organisms (especially crustaceans, molluscs, and echinoderms), but
some feed on fish, and the large manta rays (Figure 6.4b) are plankton
feeders.

Sharks and rays typically have internal fertilization and low fecundity,
producing only small numbers of relatively large eggs. Most sharks and all
rays give birth to Uve young. Skates lay their eggs in protective cases that
are attached to a substrate, and the young hatch from these within a few
weeks or months.

There is an increasing demand for shark meat and shark fins, the latter being
considered a delicacy in Asia. Numbers of sharks have sharply and rapidly
declined in regions where fishing has intensified, and many sharks are also
captured incidentally in commercial fishing operations for other species.

QUESTION 6.4 Why are shark pop u latió os likely lo be slow to recove r from
over-exploiiation?

Osteichthyes

The many species of teleost fish inhabit di verse types of marine
environments, and consequently they are a heterogeneous group in terms of
anatomy, behaviour, and ecology.

The most familiar teleosts are those that are harvested commercially
(Figure 6.5a-f; Figure 6.9) and, because of their economic importance, more
is known about the biology of these particular species. Such fish feed on
many different prey Ítems depending on their size, location, and the
availability of prey at different times. Some are strictly planktivores
(plankton-feeders), other fish are piscivores (fish-eaters), or a combination of
both. The most numerous of these fish occupy lower trophic levels; these
inelude herring, pilchards or sardines, and anchovies, all of which eat chiefly
zooplankton, although adult anchovies can feed directly on large
chain-forming diatoms as well. Larger fish, such as cod, hake and pollock,
may begin their lifes by feeding on small zooplankton; as juveniles, they
switch to preying on larger zooplankton (e.g. euphausiids), and then become
piscivores as adults. The largest of the pelagic teleosts are piscivorous
species such as tunas, jackfish, and barracuda. Some fish, such as cod,
haddock and hake, feed both in mid-water and on the sea bottom and are

Figure 6.5 Commercially harvested types of
teleost fish (a-f) and squid. (a) anchovy;

(b) capelin; (c) cod; (d) pollock; (e) halibut;
(f) tuna; (g) squid.

159

capable of catching fish or benthic invertebrates. True demersal fish spend all
their lives on or near the sea bottom, where some (e.g. solé) feed only on the
benthos (clams, worms, and crustaceans being favoured foods) and others
(e.g. halibut, turbot) eat smaller fish.

Food supplies for oceanic fish vary in abundance due to physical factors.
Some species respond to predictable seasonal variability in food
concentration by migrating to certain feeding sites when prey becomes
particularly abundant. For example, the migrations of tuna in the Pacific put
them in areas where swarms of pelagic crabs are seasonally available.
However, for many fish, variability in food concentration may cause
significant change in their growth rate and survival, and this is reflected in
the variability in fish catch from year to year.

Fish associated with special benthic habitats, such as coral reefs, are
themselves specialized to feed on coráis or resident plants and animáis; these
species are considered in more detail in Section 8.6. As with most other
animal groups, the largest fish populations are found in températe waters, but
species diversity is much higher in tropical and subtropical waters.

How do mesopelagic and balhypelagic teleosts di fíe r from epi pelagic
species?

Fish residing in deeper waters (>300 m) are not as numerous as epipelagic
species, and they are not exploited commercially. The most diverse of the
roughly 1000 mesopelagic fish species, both in numbers of species and
individuáis, are the 300 + species of stomiatoids (Figure 6.6a-c, f, g) and
the 200-250 species of lantern-fish (also called myctophids) (Figure 6.6d,
e). Bioluminescence (see Section 4.4) is common in both groups, and the
ñame Tantern-fish’ refers to light production from numerous photophores
that are arranged in specific patterns in the different species of myctophids.
Stomiatoids are distinguished by having photophores arranged in definite
rows, as well as by having the dorsal fin located far back on the body. The
photophores in both groups of fish contain symbiotic bacteria that produce
the light which may be used to lure or lócate prey, or to find mates in these
dark depths.

The majority of mesopelagic fish are small, ranging from about 25-70 mm
in length at maturity; the largest mesopelagic species are about 2 m long.
Many of the stomiatoids have elongate, relatively streamlined bodies, but the
hatchet-fish with their large, upwardly-directed eyes are named for their
laterally-flattened and squared shapes (Figure 6.6c, g). Stomiatoid fish
typically have large jaws with numerous sharp teeth, and they feed on
zooplankton, squid, and other fish. Some species have the capacity to
unhinge their jaws in order to ingest large prey (Figure 6.7), and many
species have extensible digestive organs to accommodate large food Ítems
(Figure 6.8). The best known stomiatoid genus, Cyclothone (Figure 6.6b),
contains many species, and these fish live between 200 m and 2000 m depth
in large schools. The shallower-living species are silvery or partly
transparent; the deeper residents are typically black. The lantern-fish perform
diel vertical migrations, some rising to the very surface to feed on planktonic
crustaceans and chaetognaths, and this group comprises a major food source
for tuna, squid, and porpoises. Lantern-fish range in size from 25 mm to
250 mm in length. Many display sexual dimorphism in the arrangement of
photophores, suggesting that they recognize sex by male-female differences
in light patterns.

160

Figure 6.6 Mesopelagic (a-g) and bathypelagic (h-j) fishes. (a) Vinciguerra attenuata, a
stomiatoid. (b) Cyclothone microdon, a stomiatoid. (c) Argyropelecus gigas, a hatchet-fish.
(d) Myctophum punctatum, a lantern-fish. (e) Lampanyctus elongatus, a lantern-fish.

(f) Bathophifus longipinnis, a stomiatoid. (g) Argyropelecus affinis, a hatchet-fish.

(h) Eurypharynxpelecanoides, a gulper-eel. (i) A female Ceratias holboelli with an attached
parasitic male (j), deep-sea angler-fish.

161

Figure 67 Swallowing mechanism of the
stomiatoid Chaufiodus sloani, a deep-sea viper
fish. (a) The positions of the skull and jaw
bones when the mouth is open and closed.

(b) The attitudes of the fish when ingesting
prey.

In bathypelagic waters (below 1000 m), there are about six times fewer fish
species. The greatest diversity is found in the 100 or so species of ceratioid
angler-fish, so-named because of the characteristic bioluminescent lures
which the females dangle in front of their mouths. Population sizes also
diminish in deeper water and, as potential mates become more difficult to
find, some fish (and also some invertebrates) exhibit reproductive and
development patterns that differ considerably from those of shallower-living
species. One extreme strategy has developed in some of the angler-fish in
which young males live freely, but later attach themselves to females

Figure 6.8 The ingestión capability of
deep-sea fish. (a) Chiasmodon niger with a
curled-up fish in its stomach that is longer than
itself. (b) EvermanneUa atrata containing a
squid. Sizes range up to 150 mm long.

162

(Figure 6.6i, j). The males undergo a morphological transformador! and
remain small (about 15 cm long); they Uve as an external parasite on the
much larger ( ca . 1 m long), free-living female and serve only to fertilize her
eggs. Gulper-eels ( Eurypharynx ) (Figure 6.6h) are also residents of the
bathypelagic zone. These dark-coloured, elongate fish, with a funnel-like
throat, attain lengths of 1-2 m and are capable of swallowing large fish
as prey.

In contrast to elasmobranchs, almost all teleosts have external fertilization
and high fecundity (see Section 4.3). Whereas some species attach their eggs
to a substrate, most lay large numbers of small floating eggs, and the
hatching larvae form part of the meroplankton. Teleosts typically spawn
many times, and growth is continuous through Ufe. These characteristics
make them less vulnerable to commercial harvesting than the cartilaginous
sharks and rays.

6.6.1 FISH MIGRATIONS

The swimming abilities of most epipelagic fish make them independent of
ocean currents, and they are able to migrate from one area to another,
selecting favourable conditions in terms of food availability or reproductive
sites and associated physical parameters. Whereas many species may
undertake oceanic migrations ranging from several hundred to several
thousand kilometres between, for example, feeding and spawning areas (see
Figure 6.14), other fish may undertake migrations between the sea and
freshwater.

Anadromous fish, such as salmón, sturgeon, shad, smelt and sea lampreys,
breed in freshwater. The young then migrate to sea, where they spend most
of their adult life. The length of time spent at sea is species-specific, but the
adults eventually return to their specific freshwater sites to breed and spawn.
Some species, like the Pacific salmón, die after mating; but others, such as
the Atlantic salmón, do not and may return several times to their
breeding site.

Catadromous fish are those that breed in the sea, but spend the majority of
their adult life in freshwater. Some of the longest migrations are undertaken
by the catadromous American {Anguilla rostrata) and European ( A . anguilla)
eels. Adults migrate from rivers in Europe and eastern North America to
breeding sites in the Sargasso Sea, where they spawn many small floating
eggs in deep water and then die. The larvae remain at sea for one or two
years before arriving at the coasts of America and Europe, respectively;
there they metamorphose into elvers which enter the estuaries and freshwater
rivers. They remain for 8-12 years in their freshwater habitats before
returning to the sea as mature adults.

6.7 FISHERIES AND FISHERIES OCEANOGRAPHY

Marine fisheries constitute a multibillion-dollar industry supplying about 20%
of the animal protein consumed by humans, and also producing animal feeds
for domestic livestock and poultry, fish oils for paints and drugs, pet foods,
and some food additives. As the human population continúes to expand, the
increasing demand for high-quality protein and other marine resources has
focused attention on the present stocks of commercial marine species and on

163

the feasibility of increasing, or at least maintaining, the present harvest. It
has become apparent that fisheries management has not always been
successful in maintaining fish yields and conserving stocks (see Section 9.1),
and that our information concerning the biology and ecology of many
species may be insufficient to establish reliable estimates of yields. One
branch of marine Science, fisheries oceanography, addresses these problems.

6.7.1 WORLD FISH CATCH AND FISHERIES MANAGEMENT

In an evolutionary sense, the most successful of the larger marine animáis
are the extremely abundant species of fish that are hunted commercially.
These inelude herring, anchovies, sardines, cod, and mackerel, some of
which are illustrated in Figures 6.4 and 6.9. These fish, and others, are
among the top ten species of fish that make up the world fish catch
(Table 6.2); Table 6.3 lists the major fishing nations of the world. The total
world reported catch of marine fish (including shellfish and squid) in 1993
was 84 million tonnes, down from a high of 86 million tonnes in 1989. At
least a further 27 million tonnes of by-catch - unwanted marine species
caught incidentally - is thrown back into the sea every year, most of it
dead or dying. The largest fraction (64%) of the global marine catch comes
from the Pacific Ocean, with 28% from the Atlantic and 8% from the
Indian Ocean. Humans directly consume about 70% of the total world
catch (in live weight units); the remaining 30% is used as poultry or
livestock feed, and this comes mostly from the smaller species of fish such
as anchovies, herring, and sardines (or pilchards).

Table 6.2 Principal species of fish comprising the total world fish catch (FAO
statistics, 1993). Relative dominance of species may change from year to year due to

climate change and/or exploitation pressure.

Species 1993 catch

(x 10 6 tonnes)

Peruvian anchoveta ( Engraulis ringens ) 8.3

Alaska pollock ( Theragra chalcogramma ) 4.6

Chilean jack mackerel ( Trachurus murphyi) 3.4

*Silver carp ( Hypophthalmichthys molitrix) 1.9

Japanese pilchard ( Sardinops melanostictus) 1.8

Capelin (Mallotus villosus) 1.7

South American pilchard ( Sardinops sagax ) 1.6

Atlantic herring ( Clupea harengus) 1.6

Skipjack tuna (Katsuwonus pelamis) 1.5

* Grass carp ( Ctenopharyngodon idellá) 1.5

* Freshwater and cultured species

QUESTI0N63 In Table 6.3 how can yon explain the high fish catch of two of
the world's smaller countries. Chile and Perú? ( Refer to Section 3,5 if
necessary.)

The management of the world’s fishing industry is very complicated because
it involves not only biological and econological knowledge of many species,
but it also must take into account economic considerations, competition
between nations, labour unions, and public marketing strategies. It is beyond
the scope of this text to deal with economic and political problems in
fisheries, but it is germane to consider oceanographic topics that may supply
explanations for comparative abundance of fish species and for fluctuations
in fish populations (as shown, for example, in Figure 6.9 and 6.10).

164

Table 6.3 Principal fishing nations of the world and 1993 FAO catch statistics
(including aquaculture). Relative dominance of nations may change from year to
year due to ecological or economic changes.

Country

1993 catch
(xlO 6 tonnes)

China

*17.6

Perú

8.5

Japan

8.1

Chile

6.0

U.S.A.

5.9

Russian Federation

4.5

India

4.3

Indonesia

3.6

Thailand

3.3

Korean Republic

2.6

Norway

2.6

Philippines

2.3

*Approximately 50% of the fish catch from China is from aquaculture

The history of fisheries Science has been briefly reviewed in Section 1.4. Early
studies on the population dynamics of fish stocks led to the development of
what are generally referred to as stock/recruitment theories, ‘stock’ referring
to population numbers of adult fish, and ‘recruitment’ to the numbers of
juvenile fish entering the adult population. These fisheries theories were based
upon a central premise that reproduction, survival, and productivity of fish
populations were largely independent of changes in the physical environment
of the fish, or of changes in biological components (i.e. interacting species)
within the community under consideration. The basic argument put forward
was that the recruitment of new fish stock was a function of the numbers of
eggs produced and subsequent survival of young. Because total egg
production is a function of the size of the adult population and survival was
considered constant, it was maintained that the size of the adult stock could
be controlled by manipulating fishing pressure through regulating the
number of boats, the size of nets, and the total allowable catch. This basic
premise, with later variations, became the basis for the management of
fisheries for nearly 100 years.

In cases when reliable assessments can be made of the numbers of juvenile
fish that will enter a fishery within the next one or two years, the stock/
recruitment approach has been of some valué. On the whole, however,
fisheries scientists have not been successful in managing fish stocks, or in
making long-term predictions. It is now clear that the abundance of fish is
determined by a variety of factors, and data are being compiled from a
number of fields to answer fundamental questions about fish ecology.
Plankton ecology may provide some answers, as most fish eggs and fish
larvae are meroplanktonic, and many adult fish depend on the plankton
community for food. Changes in the physical environment may also have
strong influences on fish populations. Fisheries oceanography concerns the
search for knowledge about the natural regulation of fish populations and
seeks to apply this information to fisheries management.

6.7.2 FLUCTUATIONS IN THE ABUNDANCE OF FISH STOCKS

Changes in the abundance of some species of fish are associated with long-
term changes in the oceanic climate (see Section 4.8). Figure 6.9 shows a

quantity

165

Figure 6.9 The Russell cycle in the western
English Channel: Long-term fluctuations in the
abundance oí herring ( Clupea harengus),
pilchards (Sardina piichardus), and mackerel
(Scomber scombrus) as related to changes in
oceanographic climate and the ratio oí
numbers oí the chaetognaths Sagitta elegans and
Sagitta setosa. (Arbitrary units)

regime shift, known generally as the Russell cycle, that has taken place in
the western English Channel. Herring decreased in abundance from about
1930 as water temperatures warmed. At the same time, pilchards started to
become more abundant, reaching their highest populations during the 1940s
and 1950s when herring were scarce. As water temperatures cooled in the
1960s, pilchards disappeared from the area, and mackerel became the
dominant pelagic fish. During the same periods, there was a change in the
dominant chaetognath species, from Sagitta elegans during cool intervals to
S. setosa in warmer water. It is generally agreed that this is a natural cycle
caused by climate change, and that the changes in fish species are
independent of fishing activity.

Figure 6.10 presents the history of the Peruvian anchovy catch as an exampíe
of a managed fishery that failed to take natural environmental change into
account. Waters off Perú are normally enriched by Coastal upwelling that
leads to very high productivity (see Section 3.5.2) and a short food chain
(Figure 5.3). Until about 1970, enormous numbers of plankton-eating
anchovies were produced, and these were the major food for millions of
seabirds (boobies, brown pelicans, gannets, and cormorants being dominant).
The birds are collectively referred to as guano birds, the Spanish ñame
referring to the faecal droppings of the birds. Guano from the birds built up
over time on their nesting islands to depths of up to 50 m and, because it was
rich in nitrates and phosphates, it was collected and sold for fertilizer.

The Peruvian anchovy fishery was developed in the late 1950s and was to be
a model of fisheries management. It was predicted, based on stock size, that
between 9 and 10 million tonnes of anchovies could be harvested annually
without decreasing the stock. Figure 6.10 shows the increase in annual fish
catch from less than one million tonnes in 1958 to 13 million tonnes in

166

Figure 6.10 Changes in the guano bird population and the Peruvian anchovy (Engraulis
ringens) catch along the west coast of South America. Arrows indícate El Niño years and the
relative intensity of these climatic changes. Anchoveta numbers represent the total catch
from Perú, Chile, and Ecuador.

1970, making it the largest fishery ever based on a single fish species.
However, the fishery collapsed in the 1970s and it has taken 20 years to
recover to an annual catch of about 8 million tonnes of fish (see Table 6.2).
One of the factors that was not fully understood in the 1960s and 1970s
when this fishery was being ‘managed’ was the effect of El Niño events on
the anchovy stock.

El Niño is the ñame given to a warm, nutrient-poor surface current that
flows over the coid Coastal upwelling off the coast of Perú. The phenomenon
has been well documented for about 50 years, and was recorded for
hundreds of years before that. El Niño events occurred in 1957-58, 1965,
and 1972-73 (Figure 6.10). Each time they occurred, there was a decrease in
the guano bird populations which fed on the anchovy. With the intrusión of
warm oligotrophic surface waters, the anchovy migrated deeper in the
water — too deep to be reached by these diving birds, and many birds
starved. In 1957, up to 20 million birds died during the El Niño, but
recovery of bird populations following natural El Niño events was usually
relatively rapid. However, the fishery continued during these El Niño
episodes in spite of the fact that the anchovy were being displaced from their
natural, near-surface habitat. A lack of understanding the effects of El Niño
on the anchovy, coupled with overestimates of máximum sustainable yield
as calculated from stock/recruitment theory, led to the eventual depletion of
the stock. The fishery harvested less than 2 million tonnes of anchovy from
1977 to 1985. Bird populations dropped to about one-tenth of their original
numbers and, although fish are now increasing in number, the seabirds have
not recovered from levels of about 3 million.

The examples of the Russell cycle and the Peruvian anchovy fishery have
been given to demónstrate that environmental changes may significantly
affect both the yield of fish and the type of fish present in any area. When
overfishing is coupled with a natural environmental change that is also
decreasing the stock, the consequences may be severe and the recovery of

anchovv catch (x 10 6 ton ñas)

167

the stock may take decades, if it occurs at all. It is now clear that the role of
the environment cannot be excluded from fisheries theories, and that
oceanographic data may contribute useful and necessary information to
create new management theories. Because marine fish stocks occupy large
expanses of ocean, satellite remóte sensing is now being applied to examine
those variations in ocean conditions that cause natural fluctuations in the
distribution, abundance, and availability of commercial stocks.

6.7.3 REGULATION OF RECRUITMENT AND GROWTH IN FISH

Most teleost fish species are highly fecund, each female usually spawning
between 10 3 and 10 6 eggs per year. An extremely small variation in the
mortality of the progeny (e.g. from 99.90 to 99.95%, see Question 6.11) may
cause a very large change of several hundred per cent in the adult population
size. Thus it is important that fisheries management theories inelude an
ecological understanding of the factors controlling the survival and
recruitment of larval and juvenile fish into adult stocks. Factors determining
growth rates of fish are also important because they determine size and
partly control survival in young fish.

A number of hypotheses have been formulated to explain fluctuations in
adult abundance due to differences in the recruitment and growth of young
fish, and none are mutually exclusive. Some of these hypotheses are listed as
folio ws:

1. Starvation hypothesis. If there is not enough planktonic food in the
sea, larval fish mortality will increase and few, if any, will survive to
become adults.

2. Predation hypothesis. Predators, including larger fish and some
carnivorous zooplankton, may consume large numbers of both larval and
juvenile fish. Heavy predation results in few young surviving to become
adults.

3. Advection hypothesis. Physical oceanographic processes may transport
the young fish away from their nursery areas to unfavourable environments
where they will not survive.

4. Growth hypothesis. The máximum size attained by fish at the time of
harvest, multiplied by the number of fish captured, gives the biomass yield
to the fishery. Size and numbers of fish are also used to establish fish quotas
in terms of allowable tonnage. Numbers are determined by survival (see
hypotheses 1-3 above), and size is determined by growth. The growth
hypothesis is based on the consequences of fish growth being inhibited by
either biotic (e.g. food) or abiotic (e.g. temperature) factors.

In order to see how these hypotheses might be integrated with present
biological oceanographic knowledge, several mechanisms of fish recruitment
and growth are discussed below.

The tendeney of teleost fish to produce very large numbers of eggs is quite
unlike the reproductive patterns of most terrestrial animáis of similar size. A
single female cod, for example, may lay more than a million eggs per year.
Obviously only a very small percentage of these eggs are needed to replace
the adult stock. The mortality of eggs and fish larvae is very high, as
illustrated in Figure 6.11, and may result from predation, advection, or, in
the case of larvae only, from starvation.

168

Figure 6.11 Idealized population mortality and
individual growth curves for a species of teleost
fish during its lite cycie from egg to adult.
Arbitran/ units of age and length, and variable
time intervals (indicated by dashed lines) for life
stages.

1 The hypothesis that starvation may regúlate larval fish survival is
concerned with what is calleó the critical phase in the life of fish. This
phase begins immediately after hatching when the young larvae still have the
remnants of the yolk sac on which to subsist. In order to survive, the larvae
must begin to eat sufficient planktonic food before the yolk is exhausted.
This means that the larvae must hatch at a time in phase with abundant
plankton concentrations. If a larval fish hatches too early, or too late, relative
to its food supply, it will die (Figure 6.12).

2 The predation hypothesis assumes that larger organisms have fewer
predators and are also better able to escape from predatory attacks, Thus if
larval or juvenile fish can grow fast enough, mortality will be lessened and
more fish will survive. This is illustrated in Figure 6.13 which also shows
that this hypothesis is partly dependent on the planktonic food supply,
although here food concentrations are not restricted to the critical phase as in
Figure 6.12.

QUESTION 6.6 (a) Referring to Figure 6.13b, is ihere a growth rale for young
fish below which there would theoretically be 100% mortality due to
predation? (b) Would you ex pee t to find 100% mortality from predation in
nal u re?

3 The advection hypothesis can be illustrated in a number of different ways
depending on the type of fish species. For example, plaice tend to spawn on
specific sites that are associated with favourable nursery areas to which the
larvae are carneó by cunents. This is shown in Figure 6.14 for a population
of plaice that spawns in the Southern North Sea. In some years, however,
strong storm activity may disrupt the current System that canies larval fish to
their nursery area, and the plaice larvae may then be transponed to areas that
are unfavourable for survival.

4 The growth hypothesis derives from the growth curve shown in
Figure 6.11. Growth is dependent on a variety of parameters affecting the

Figure 6.12 The critical phase of larval fish survival requires that planktonic food (often
copepod nauplii) must be present in the water at the time of hatching (e.g. at time A), it the
food organisms occur later (e.g. at time B), all the fish larvae from one particular spawnmg
will die from starvation. (Arbitrary units.)

time

169

(a) (b)

Figure 6.13 (a) A hypothetical relationship showing the difference in growth rates (2% to
8% per day) of juvenile fish depending on the concentraron of planktonic food (usually,
copepods).

(b) The growth rates from (a) are shown over time, together with the size at which maximal
predation of the fish takes place. Note that this size also increases slightly with time as the
predators themselves grow. Juvenile fish that grow slowly (e.g. at 2% day" 1 ) are exposed to
predation over a longer time period and therefore have lower survival rates than
faster-growing (e.g. 8% day -1 ) fish. (All units are arbitrary valúes.)

Figure 6.14 The larval drift and migrations oí
plaice (Pieuronectes platessa) in the North Sea.
Larvae hatch in the spawning area, then drift
northeast to a food-rich nursery area. They
remam in the nursery area for their first year of
Ufe, growing from 1.5 cm to 20 cm in length
before migrating to the northwest.

rate of growth and the length (size) at maturity. In general, the growth rate is
directly proportional to temperature, but size at maturity is inversely
proportional to temperature. Thus an increase in temperature, which is
governed by the physical ocean climate, can have the dual effect of
producing more rapidly growing fish, but ones that are smaller at maturity.

In addition, the growth efficiency (K\ or K 2 , see equations 5.10 and 5.11) of
fish varíes with the type of food consumed. Prey with high protein content
(e.g. copepods) produce faster growth than foods (e.g. small ctenophores)
with very high water content and low protein. Growth is also influenced by
the metaboiic costs associated with particular types of prey; for example, a
predator that has to chase its prey would have a higher metaboiic cost than
one that filters its food. Thus, as prey type changes (due to changes in the
ecosystem), the growth efficiency of fish will also change, and this will
affect the growth curve (Figure 6.11).

In summary, each of the four hypotheses discussed above can be shown to
have some experimental support. However, none can be shown at present to
be the only mechanism determining the fluctuations in the abundance of fish,
and it is likely that more than one mechanism is operable. It is also possible
that some other factors, such as fish disease, may at times be important in
regulating recruitment of young to adult stocks. In order to improve the
management of fisheries, these mechanisms need to be further researched
through experimentation, field data, and ecosystem Computer models.

QUESTIQN 6.7 Does Figure 6.11 suggest that. in order to increase the biomass
yield of fish, it is better for a fishery lo al low larger numbers of small fish to
survive, or smaller numbers of large fish? What other factors might affect
the decisión?

170

6.7.4 FISHING AND THE USE OF NEAR REAL-TIME OCEANOGRAPHIC DATA

The previous section has concentrated on mechanisms that might be relevant
in producing seasonal and long-term variations in the abundance of fish.
Another problem faced by fishermen is where to find the main concentrations
of fish. In general, fishermen have been almost too successful in finding fish,
and some stocks have been seriously depleted due to the massive investment
in the mechanized harvest of fish. However, it still remains a problem to
forecast the exact location of fish schools on time scales that relate to the
time which a fishing boat can economically spend at sea.

Certain types of oceanographic data, such as surface temperatures and depth
of the thermocline, can greatly assist fishermen in rapidly locating fish and
in reducing the cost of remaining at sea, but only if these data are collected
during a time that is near to the harvest period. This type of information is
referred to as ‘near real-time data’; the data are collected very cióse to the
time of an event for which the data are required.

Fish become easy targets for fishermen when they are concentrated into
schools. They may form schools when feeding, or during periods of
reproduction and migration. Feeding schools are often associated with very
productive waters, such as might be present on a particular bank (i.e. a
shelf-break frontal zone as defined in Section 3.5.4), or at a boundary
between two water masses (e.g. a planetary frontal zone, Section 3.5.3).
Reproduction may take place in other localities, with the fish migrating
together between feeding and spawning sites.

The location of particular types of fish is often an important factor in the
regulation of fisheries. For example, in the North Pacific Ocean, salmón
generally stay in subarctic waters of <14°C, whereas the squid and tuna
fisheries are in subtropical waters of >14°C. The boundary between these
two water masses is not a fixed line, but varies geographically by several
hundred kilometres depending on the physical oceanography. Near real-time
oceanographic information on the location of this frontal zone is important
to the high seas fisheries. This is particularly true for the squid fishery which
is carried out in the open ocean and, by international agreement, is not
allowed to stray into areas where salmón may be inadvertently collected as
well. Ocean surface temperatures can be detected by satellites, and the
location of the 14° boundary is relayed to the Japanese squid fishing vessels
via a land-based station which also monitors the location of the fleet.

It is also important for inshore fixed-depth fisheries to be able to predict the
location of fish schools from near real-time oceanographic data. Figure 6.15
illustrates the location of cod stocks relative to a trap fishery that is
maintained at a fixed depth throughout the year. (This illustration serves as a
good example because the location of the fishing effort is fixed every year;
however, it could also apply to a net fishery operating at a fixed depth.) A
trap, consisting of a large net that directs swimming fish into a central area
from which they cannot escape, is set from the surface to the seafloor.
Various physical disruptions in the water temperature can result in the
location of the cod at very different depths and regions relative to the trap
locations. Under some circumstances (conditions b and d in Figure 6.15), the
position of the traps is such that few, if any, fish would be captured. At such
times, fishermen might profitably employ a different type of fishing method,
using nets towed from boats or, in the case of condition ‘d\ the traps might
be relocated in deeper water if feasible. The application of near real-time

171

Figure 6.15 The trap fishery ¡llustrated here (and other inshore fisheries) can be severely
affected by local water temperature conditions, which in turn are determined by prevailing
wind and air temperature conditions. (a) shows a June situation in an area having a late,
coid spring with light variable prevailing winds. The surface water has warmed enough to a
depth of about 18 m to allow cod to come cióse inshore at shallow trap depths. In
(b), prevailing offshore winds have driven the suitable water layer off the shore, and coid
water coming to the surface severely restricts the area in which trap fishing might be
successful. In (c), prevailing onshore winds early in the season have had the opposite effect,
pushing the suitable water layer onto shore, and deepening and expanding the potential for
good trap fishing. Finally, in (d), a late-season situation is shown in which prolonged warm
weather and onshore winds have combined to produce a nearshore layer of water too warm
for cod; the fish stay below it out of reach of the trap.

data on the thermal structure and movement of the water masses near the
traps could greatly assist this fishery, and such information should become
an integral part of fisheries management.

As mentioned earlier, fish schools also form during reproductive periods, and
the congregation of herring on certain banks in the North Sea is a
well-known phenomenon associated with the spawning of these fish.
Unfortunately, it has also become the time and place where the herring are
most easily caught — just when they are about to create a new population.
There is obviously a división of opinión in such a case as to whether one
catches the fish when it is most economical to do so, or whether it would be
better to wait for a time when less biological damage would be done.

The migration routes of fish can also be greatly influenced by local
oceanographic conditions, which in turn affect the fishery. An example from
North America is given in Figure 6.16, illustrating migration routes of Fraser
River sockeye salmón off the west coast of Cañada. During 1955-77, most
of the fish returned to the river through the Strait of Juan de Fuca, where
they were exposed to both Canadian and U.S. fishermen. However, during
1978-83, up to 80% of the fish returned through routes north of Vancouver
Island, where they were available only to Canadian fishermen. This resulted
in a change of considerable economic impact to the fisheries of the two
adjoining nations. A Computer model that used the temperature and salinity
of Coastal water (the latter indicating amount of freshwater inflow from
rivers) was developed to predict the diversión in 1978-83 as indicated in the
inset graph.

172

Figure 6.16 The migration routes of sockeye
salmón returning to the Fraser River (British
Columbia, Cañada) for spawning. The inset
graph shows the percentage of fish that were
diverted through Johnstone Strait during return
to the Fraser River from 1955 to 1988 (solid
line). From 1955 to 1977, most fish returned to
the river through the Strait of Juan de Fuca;
from 1978 to 1983, the majority of the salmón
returned through Johnstone Strait. The dashed
line in the inset graph indicates predicted
diversions through Johnstone Strait based on a
Computer model simulating changes in
freshwater flow and water temperature.

6.8 MARICULTURE

One way of managing fishery resources and increasing yield is to attempt to
control many environmental variables by growing marine species in
enclosures or impoundments. Such systems, because they concéntrate the
target species and are located in Coastal areas, also have the advantage of
easy and economical harvesting. The cultivation of marine species is called
mariculture. It can be regarded as the marine counterpart of agriculture, as
many of the principies and problems associated with increased production
through artificial culture are similar. Interest in mariculture continúes to
increase as the human population expands and demands more protein, and as
open-sea fishing reaches the limits of exploitation of wild stocks.

A variety of pelagic and benthic organisms are cultured including some
seaweeds, crustaceans (shrimp or prawns), molluscs (mussels, oysters,
scallops, clams, and abalone), and fish (e.g. salmón, mullet, solé, turbot,
eels). Most of these species are raised for human consumption, but some are
cultured for other commodities, such as pearls, food additives (e.g. alginates
from seaweeds), and domestic animal feed.

One of the simplest forms of mariculture involves the transplantation of wild
stocks (e.g. fish, oysters) into new areas where natural conditions are
favourable for increased production. Successful transplants have involved the
introduction of a Pacific salmón into Lake Michigan in the U.S. and into
New Zealand and Chile. Another very simple technique is the trapping of a
wild population in enclosures, where the animáis are held without artificial
food until harvesting; this is done with shrimp in Singapore and some fish
(e.g. mullet) at various localities. More intensive culture requires more
manipulation in terms of providing feed and fertilizers, controlling the
physical environment, and eliminating predators and disease.

173

The most economical species to culture are those that occupy lower trophic
levels, feed on naturally available food, and can produce high biomass in
crowded conditions. Mussels, for example, feed by filtering plankton, and
they are routinely cultured in many Coastal areas. In nature, mussels attach to
hard substrates and their numbers are often controlled by the amount of
space available and by the degree of predation caused by starfish and boring
snails. In culture, mussels are provided with ropes suspended from rafts for
attachment, thus increasing the space available and also eliminating benthic
predators; the mussels feed on plankton in the surrounding water. Such
intensive culture can yield up to 600 tonnes of mussels per hectare annually;
about 50% of this is drained body weight, available for consumption.

Animáis from higher trophic levels require the provisión of either artificial
feed or of cultivated or captured prey and, because they are generally larger
in size and mobile, they also occupy more space. However, the relatively
high costs of culture may be justified by higher market valué. For example,
salmón can be grown in enclosures (Figure 6.17) and fed on synthetic feed
(which has the advantage of chemically known constituents) or on processed
euphausiids captured at sea (see Section 6.1); the high expenses of this
culture are recovered in the high market price of the salmón.

Another form of salmón enhancement is to rear salmón eggs in hatcheries
and then release the young salmón into the sea. This is known as ocean
ranching, from analogy with raising some cattle by releasing them into
ranchlands. Depending on the species, released salmón spend between 2 and
5 years in the open ocean before returning to the same stream in which the
hatchery is located. Ocean ranching is less expensive than pen culture, but
the retums of adult fish depend upon survival at sea.

The examples given above all utilize only one species; this type of System is
known as monoculture, and it represents an extreme simplification of a
natural community. In order to increase profitability, some Systems attempt
to culture more than one species in the same enclosure; this is known as
polyculture. In a monoculture system, uneaten food and faecal material fall
to the bottom where they are either flushed by natural water movement, or
are otherwise cleaned from tanks to prevent excessive bacterial growth and

Figure 6.17 An aerial view of a salmón farm
in British Columbia, Cañada. The salmón pens
are located in a protected cove that shelters the
enclosures from storm damage.

174

the formation of anoxic water. A polyculture system introduces species that
feed on this detritus and can also be harvested for commercial profit.
Herbivorous fish (e.g. some carp, mullet) are commonly grown in ponds
together with prawns that feed on detritus, fishmeal, and/or filamentous
algae. It is also possible to combine fish that feed on zooplankton with those
that eat benthic plants. Polyculture is presently more highly advanced in
freshwater systems, but it can be expected to expand as more mariculture
systems are developed.

The leading countries in mariculture tend to be those with high human
populations and high protein demands. Fish (carp) culture began about 4000
years ago in China, and this country continúes to lead the world in terms of
quantity of production from combined marine and freshwater culture (see
Table 6.3). The second leading country is Japan, which has developed some
of the most advanced mariculture facilities and techniques. In North
America, only about 2% of fishery producís come from culture, and that
figure ineludes cultivated freshwater species as well as marine organisms. In
Europe, Spain is the leading producer with mussel culture. On a global basis,
roughly 5 x 10 6 tonnes of marine species are grown in culture annually.

PUESTION 6.8 What is the perceniage of marine species produeed in culture
compared with the total marine fish catch harvested from the oceuns?

There is no doubt that mariculture will continué to expand throughout the
world in response to increasing demands for more protein for human
consumption and, to a lesser extent, to supply luxury foods such as salmón
and lobster, or other commodities. There are several constraints to this
expansión, however. Some problems are technological, like the selection of
suitable sites or disease control, and some of the constraints are economic,
balancing high costs of feed, fertilizer, and manpower against market prices.
A much more serious problem looms with increasing Coastal pollution
throughout the world. Mariculture depends on Coastal sites, whether cultured
species are grown in enclosures in the natural environment or in land-based
facilities that depend on water pumped from a nearby marine source. Eggs
and young stages of marine animáis are particularly sensitive to pollutants,
and adults may accumulate Chemical or biological substances from polluted
water that make them dangerous to humans. Shellfish, for example, filter the
cholera bacterium from sewage-polluted water, and consumption of infected
animáis spreads and compounds this disease.

6.9 SUMMARY OF CHAPTER 6

1 The nekton comprises the larger, pelagic, marine animáis whose
swimming abilities are such that their movements are independent of ocean
currents. Included in this category are larger crustaceans (some euphausiids,
shrimp, and swimming crabs), squid, sea snakes, marine turtles, and marine
mammals, with adult fish making up the dominant fraction. Seabirds are also
considered here because they are dependent on the sea for food and may
have considerable influence on the neuston and epiplanktonic communities.

2 Few commercially harvested crustaceans are pelagic, but some of the
larger, very abundant euphausiid species are presently fished in the Antarctic
and off Japan. The superabundance of the Antarctic krill (.Euphausia

175

superba ) makes it an attractive fishery target, and it is likely that this harvest
will increase, despite high economic costs.

3 Squid form another abundant invertebrate group targeted by fisheries.
More needs to be known about the biology and abundance of these animáis
before fisheries management can be effective in protecting these stocks. It
has become evident that driftnet fishing for squid is very unselective, and
that vast numbers of seabirds, fish, turtles, and marine mammals have been
inadvertently captured and killed by this method. More selective, alternative
squid-fishing techniques are available, although their use increases the costs
of fishing.

4 Eight species of turtles, one lizard, and about 60 species of snakes are
the only reptiles to have become marine. The turtles have become
endangered species from hunting of the adults and their eggs.

5 There are about 110 species of marine mammals. The largest are the
baleen whales that feed by filtering zooplankton or fish (or benthic
invertebrates, in the case of the grey whale) through their plates of baleen.
The toothed whales (including dolphins and porpoises) are predators in the
sea. These two groups of cetaceans together consume a much greater
quantity of marine biomass than is removed from the oceans by the entire
commercial fishery.

6 Many species of whales, pinnipeds (seáis, sea lions and walruses), and
sirenians (manatees and dugongs) have been extensively hunted. Their low
fecundity and long development times from birth to maturity make them
especially vulnerable to rapid depletion of population numbers through
commercial harvests. Many of the species are now endangered, and recovery
of populations is slow.

7 The most highly adapted of the seabirds spend 50-90% of their lives at
sea, but all remain dependent upon land for nesting sites. The highest
numbers are found in association with very productive waters, where
zooplankton and fish are concentrated. Non-migrating species are subject to
natural mortalities caused by climate change and subsequent declines in their
prey (e.g. the effects of El Niño on the guano birds off Perú).
Human-induced mortalities such as overfishing, habitat destruction,
introduction of predators, and Coastal pollution increasingly threaten seabirds.

8 The great majority of marine fishes are teleosts with a bony skeleton, and
the 20000 or so species show considerable diversity in terms of anatomy,
behaviour, and ecology. The most abundant species are epipelagic
plankton-feeders with very high fecundity, and many of them (e.g. herring,
sardines, anchovies) form the basis for some of the most profitable marine
fisheries.

9 There are fewer deep-water species of fish, and they are not as numerous
as their shallower-living relatives. Many of the meso- and bathypelagic fishes
tend to be relatively small. Many of the species have photophores and use
bioluminescence to lócate or lure either prey or mates, or to evade predators.

10 In 1993, the total world catch of all marine species of fish (including
squid and shellfish) was about 84 million tonnes per year. About 64% of this
catch was taken from the largest of the oceans, the Pacific Ocean, with
China, Perú, and Japan being the leading fishing nations.

176

11 Although various attempts have been made in the last 100 years to
manage fisheries, these have met with little success. Many fish stocks or
fishing regions have been depleted or are in danger of being overfished.

12 Fisheries management has traditionally been based largely on
stock/recruitment theories that have ignored the role of the environment in
causing natural fluctuations in the numbers of fish. It is becoming
increasingly apparent that the recruitment of larval fish into adult stocks can
vary greatly depending on whether there is sufficient food for young fish,
whether predation is high or low, whether larval fish are transponed by
currents into unfavourable habitats, whether disease affects the population,
and whether growth is slowed or hastened by temperature, food availability,
or other factors.

13 Biological oceanography can assist fisheries in two ways. The first way
is by increasing our understanding of what factors cause natural fluctuations
in the abundance of fish. The second way is in providing near real-time
oceanographic data in connection with the actual process of fishing; better
information on the location of fish schools can reduce the cost of the
fisheries.

14 Mariculture is another way of increasing the yield of fisheries resources.
At present, only a few of the many marine species are under culture, but
cultivation is expected to expand from its present production of about

5 x 10 6 tonnes per year.

Now try the following questions to consolídate your understanding ofthis
Chapter.

QUESTION 6.9 From a knowledge of primary productivity in re I ai ion to the
physical movement of water (Section 3.5), what other locatíons in the oeeans
in addilion lo those discussed in ihis Chapter might support good fishing?

QUESTiOM 6.10 Can the total tonnage of ‘fish’ be increased by caiching
smaller and smaller fish. or even by harvesting zooplankton?

0UEST10N 6.11 Suppose eaeh fem ale fish in a particular species lays 10' eggs
each year, In one year, 99.90% of the eggs or progeny die; in another year,
the mortaiiiy is 99,95%. Assuming that in each case the remainder beca me
mature adults, what would be the difference in the numbers of the adult
populations resultíng from these two year classes?

OüESTION 6.12 What might be some ecológica! consequenees of fisheries
cominuing lo remove increasingly large numbers of fish and squid from the
oeeans?

Q0ESTI0N 6.13 Other than selecting species for culture on the basís of trophic
level posilion, what additional physiologtcal and biological lealares might
make certain organisms more amen able and attractive as possi hie culture
organisms than others? Considcr the requirements of al! lile stages in
fonrnjlating your answer.

QUEST10N 6.14 What percentage of the world's ocean is now a designated
whale sanctuary? íCónsul! Table 5A.)

CHAPTER 7

BENTHOS

Relative to the pelagic zone, the seafloor presents a greater variety of
physically diverse habitats that differ from each other in terms of depth,
temperature, light availability, degree of immersion (tidal vs. subtidal), and
type of substrate. Hard, rocky substrates provide sites of attachment for
sessile species like barnacles and mussels which remain in one place
throughout their adult life, and they provide crevices and depressions that
can be used by mobile animáis as refuges from predators. Soft-bottom
substrates (e.g. mud, clay, sand) offer both food and protection for
burrowing animáis. At least partly owing to the greater variety of benthic
habitats, the number of species of benthic animáis (estimated at > 1 million)
is much greater than the combined number of pelagic species of larger
zooplankton (about 5000), fish (>20000) and marine mammals (ca. 110).

As in the pelagic environment, vertical gradients of temperature, light, and
salinity are especially important in establishing distinctly different living
regimes for benthic organisms. Figure 1.1 shows the ecological divisions of
the seafloor based on depth and topography. Some of the ecological-depth
divisions have well-defined boundaries, others are more arbitrary zones, but
each of these benthic habitats presents distinctly different living conditions.
The animáis that inhabit different zones will generally be of different species,
each uniquely adapted to the particular environment in which it is found.

The smallest benthic zone (Figure 1.1) is the supralittoral or supratidal
zone, an area just above high water mark and immersed only during storms.
On steep shores, this zone will receive spray from breaking waves, and it is
sometimes referred to as the ‘splash’ zone. On fíat beaches, the area may be
marked by heaps of seaweeds cast ashore. Few species are adapted to live in
this transitional región between the sea and land.

The littoral or intertidal zone lies between tide marks and is thus immersed
at high tides and exposed at low tides. The extent of this zone depends upon
local topography and tidal range. This area lies within the euphotic zone, and
benthic algae as well as phytoplankton are available for grazing and
filter-feeding benthic herbivores. These in turn support a diverse and
abundant carnivore community.

The sublittoral (subtidal) zone extends from the low tide mark to the outer
edge of the continental shelf, at a depth of about 200 m. Part of the
sublittoral area also lies within the euphotic zone, but benthic plants decline
from low numbers to zero in the deeper regions. Rocky substrates become
scarce and are replaced by soft substrates. The sublittoral zone occupies
about 8% of the submerged seafloor.

The remaining benthic habitats are located below the euphotic zone. The
bathyal zone extends down the continental slope from 200 m to 2000 or
3000 m (the lower boundary is indefinite), and it occupies approximately
16% of the submerged seafloor. The abyssal zone, extending from 2000 or
3000 m to 6000 m, is by far the largest ecological región, encompassing
almost 75% of submerged benthic habitats. This zone is also characterized
by having a temperature of 4°C or less. The deepest areas of the sea are the
trenches, extending downward from 6000 m to somewhat over 11 000 m
depth; ecologically, this benthic habitat is referred to as the hadal zone

(from ‘Hades’, the Greek mythological underworld). This last zone is the
least well known because of its inaccessibility, and relatively few species
have so far been described from it.

The vast majority of larger benthic species live in depths less than 200 m,
and there are many more species in shallow tropical waters than in shallow
coid seas. Some of the different types of shallow-water and deep-sea benthic
communities are considered in more detail in Chapter 8.

7.1 BENTHIC PLAÑÍS

A variety of marine plants attach to the seabed or live within sediments in
shallow depths. All are restricted to the euphotic zone; that is, they are
confined to intertidal and shallow subtidal regions.

Certain intertidal marine communities are dominated by large angiosperms
(flowering plants) that only flourish in sheltered regions, where accumulated
sediments allow rooted plants to develop. These communities inelude
tropical mangrove swamps, with a variety of salt-tolerant trees and shrubs
(see Section 8.7); estuarine saltmarshes which are dominated by
marshgrasses (see Section 8.5); and meadows of seagrasses which occur low
in the interidal zone. All of these benthic macrophytes (large, visible plants)
are highly productive, but they contain a large proportion of materials that
are indigestible to most marine animáis. They therefore form large amounts
of detritus, which may be exported by tidal currents into other marine areas,
and this decomposing detritus contributes to the high productivity of
Coastal waters.

Marine macrophytes also inelude the conspicuous algae that are most
abundant on rocky shores in températe zones. The algae (which are rootless)
have developed anchoring structures called holdfasts. Among the important
members of this group in terms of production are the long-stemmed kelps
(brown algae) (Figure 7.1c and Figure 8.3) that anchor to rocky substrates in
the subtidal zone (see Section 8.3). Some kelp (e.g. Macrocystis ) have
extremely fast growth rates and form large underwater forests. Other types
of seaweeds (Figure 7.1) inelude the common macrophytic algae (e.g.

Fucus) that cover rocks in the intertidal zones; they can be very abundant,
but their rate of production is usually only about half that of the kelps. Kelp
and seaweeds may be a direct source of food for some herbivores, and they
also form abundant detritus that is ultimately consumed by detritivores. As
much as 30% of their production may be lost in exudates that contribute to
the pool of DOM (see Section 5.2.1).

Some green algae (e.g. Halimeda) and red algae (e.g. Lithothamnion) have
the ability to incorpórate calcium carbonate in their tissues, which is a most
effective defence against being eaten by herbivores. These hard coralline
algae grow as encrustations over rocks or shells or coral reefs, and they can
contribute materially to the formation of carbonate deposits.

Epiphytic algae (e.g. Ectocarpus) grow on the surfaces of other larger plants
(seaweeds, kelp or seagrasses). The epiphytes are generally thin-walled and
filamentous, and therefore can be easily consumed by marine herbivores.

The least obvious benthic producers are the unicellular algae that live on
sand grains (epipsammic species), or that form mats on the surface of mud.

179

Figure 7.1 Examples of seaweeds.

(a) Enteromorpha (up to 500 mm); (b) Uíva (up
to 250 mm); (c) Alaria (up to 2 m);

(d) Chondrus (up to 150 mm); (e) Gigartina (up
to 200 mm); (f) Delesseria (up to 250 mm);

(g) Fucus vesiculosus (up to 1 m). (a) and (b)
are green algae; (c) and (g) are brown algae;
and (e) and (f) are red algae.

These microphytes (microscopic plants) inelude motile pennate diatoms,
blue-green algae, and dinoflagellates (see Section 3.1 for general
descriptions). These organisms are often extremely abundant and despite
their small size, they are an important source of primary production in
shallow waters. Certain dinoflagellates have even taken up residence within
the tissues of benthic animáis; the best known are the symbiotic algae of
coráis that are described in Section 8.6.

The rock-like reefs called stromatolites (Colour Píate 30) that are located in
shallow waters off western Australia and around the Bahamas are of great
interest from an evolutionary perspective. They are formed by mats of
microphytic photosynthetic Cyanobacteria (see Section 3.1.3) that deposit
calcium carbonate which builds up in successive layers at a rate of about
0.5 mm per year. Although the present reefs began to form relatively
recently in geologic time, the constituent Cyanobacteria are similar to
microbes that flourished two billion years ago, and thus they represent one
of the longest continuous biological lineages known.

Different benthic plants characteristically occupy different tidal levels, and
this zonation is partly determined by their different abilities to absorb
particular wavelengths of light. The complete spectrum of visible light is
available at the sea surface, but different wavelengths are quickly absorbed
and scattered within the water column (Figure 2.4). Green algae (e.g. Ulvd)
typically grow in shallow water, and their pigments absorb both long and
short wavelengths (Figure 3.4a). Brown and red algae also contain green
chlorophyll, but they have particular accessory pigments that mask its
colour. Compared with green algae, brown algae (e.g. kelp, Fucus) are most

180

abundant in somewhat deeper water; their main pigment, fucoxanthin, is
more efficient at capturing blue-green light (Figure. 3.4b). Some red algae
(e.g. Gigartina) are characteristically subtidal; their red pigments
(phycoerythrin and phycocyanin) are also efficient at absorbing subsurface
light that cannot be absorbed by chlorophyll a (Figure 3.4b). There are
numerous exceptions to this depth-distribution pattern, however. For
example, certain red algae (e.g. Porphyrá) may be found in the high
intertidal zone, and some green algae (e.g. Ulva) may occupy lower regions.
This is because other factors, such as resistance to wave action, tolerance to
drying during tidal exposure, and selective grazing by herbivores also
determine the position of plants in intertidal areas (see Section 8.2.1). In the
sediment surface, blue light is absorbed first and red light penetrates the
farthest; thus small algae growing within sand or mud may also show a
different distribution from that described above.

QUEST10N 7,1 Why are algae commonly grooped ín colour categories, such
as 4 red' algae, *browiT algae, and 'green' algae?

7.1.1 MEASUREMENTS OF BENTHIC PRIMARY PRODUCTION

Production of small benthic plants can be measured by carbón or oxygen
exchange, or estimated from chlorophyll concentration as described in
Section 3.2.1. Production of benthic macrophytes, however, is commonly
measured by harvesting and weighing the plants, with production being
reported in terms of carbón per unit area. In températe areas where seasonal
growth must be taken into account, harvesting is usually done at the period
of máximum biomass; some allowance is made for estimating the biomass
that has grown, but died and disappeared before and after the measurement.
Growth rates of fronds of macrophytes can sometimes be obtained by
punching holes in the plants; the hole will move away from the meristerm
(growing región) of the plant, and growth can be expressed in terms of
distance moved and incrase in the size of the hole.

Attached plants have the advantage of being washed with turbulent water
that brings a continual renewal of dissolved nutrients. Nutrient
concentrations are often elevated in Coastal waters, and the rates of nutrient
uptake are generally high in benthic plants. Usually the productivity per unit
area of large attached algae is an order of magnitude greater than that of
phytoplankton. Valúes for benthic productivity are given throughout
Chapter 8, where specific types of communities are considered in more
detail. Although benthic photosynthetic production exceeds that of
phytoplankton in the water column in many Coastal areas, only a small
fraction of the seafloor receives sufficient light to support attached plants.

On a global scale, production by benthic plants accounts for less than 10%
of the total primary production in the sea.

7.2 BENTHIC ANIMAIS

Benthic animáis (or zoobenthos) are separated into two ecological categories
based on where they live relative to the substrate. The infauna are those
species that live wholly or partly within the substrate; this category includes
many clams and worms (polychaetes) as well as other invertebrates
(Figure 7.2). Infaunal species usually domínate communities in soft

181

Figure 7.2 Representative ¡nfauna, showing their burrows and living positions.

(a) Hydrobia, a snail; (b) burrow of Pygospio, a polychaete; (c) burrow of Corophium, an
amphipod; (d) Arenicola , a polychaete; and the clams (e) Cardium, (f) Macoma,

(g) Scrobicularia , and (h) Mya.

substrates, and they are most diverse and abundant in subtidal regions. There
are a few infaunal species in hard substrate communities as well, rock-boring
clams being one example. The epifauna (Figure 73) are those animáis
living on or attached to the seafloor; about 80% of the larger zoobenthos
belong to this category. A few common examples of epifauna inelude coráis,
barnacles, mussels, many starfish, and sponges. Epifauna are present on all
substrate types, but they are particularly richly developed on hard substrates,
and they are most abundant and diverse in rocky intertidal areas and coral
reefs. A third category can be added to inelude those animáis that live in
association with the seafloor but also swim temporarily above it; animáis
such as prawns and crabs, or flatfish such as solé, form the epibenthos.

polychaeíe

tubeworm

Figure 7.3 Representative epifauna and epiflora.

182

It is also convenient to classify benthic animáis into size categories. In this
case, size is relative to the mesh size of sieves used to sepárate animáis from
sediments. The following categories encompass all sizes of benthos:

Macrofauna (or macrobenthos): those animáis retained by a
1.0-mm-mesh sieve. These are the largest benthic animáis, including starfish,
mussels, most clams, coráis, etc.

Meiofauna (or meiobenthos): those animáis retained by a
O.M.0-mm-mesh sieve. These are small animáis commonly found in sand or
mud. The group ineludes very small molluscs, tiny worms, several small
crustacean groups (including benthic copepods), as well as less familiar
invertebrates (see Section 8.4.2 and Figure 8.5).

Microfauna (or microbenthos): those animáis that are smaller than
0.1 mm in dimensión. This smallest size category is largely made up of
protozoans, especially ciliates (Figure 7.4).

7.2.1 SYSTEMATICS AND BI0L0GY

Benthic communities contain an extremely diverse assemblage of
zoobenthos. Many of these marine species have no terrestrial or freshwater
counterparts and are unfamiliar animáis. Some of the dominant types of

Figure 7.4 Examples of microfauna: ciliate protozoans showing diversity of form.

183

Table 7.1 Major taxonomic groups and representatives in marine benthic
communities.

Phylum

Subgroups

Common names/representatives

Protozoa

Foraminifera

forams

Xenophyophoria

—

Ciliophora

ciliates

Porifera

sponges

Cnidaria

Hydrozoa

hydroid polyps

(formerly

Anthozoa

sea anemones; coráis

Coelenterata)

Platyhelminthes

Turbellaria

flatworms

Nematoda

roundworms

Nemertea

ribbon worms

Annelida

Polychaeta

polychaete worms

Pogonophora

beard worms

Vestimentifera

vestimentiferan worms

Sipuncula

sipunculids (peanut worms)

Echiura

echiurids (spoon worms)

Hemichordata

Enteropneusta

acorn worms

Mollusca

Gastropoda

snails; nudibranchs

Bivalvia

clams; mussels

Polyplacophora

chitons

Aplacophora

aplacophorans

Scaphopoda

tusk-shells

Cephalopoda

octopuses

Echinodermata

Asteroidea

starfish

Ophiuroida

brittle stars

Echinoida

sea urchins; sand dollars

Holothuroidea

sea cucumbers

Crinoidea

feather stars; sea lilies

Ectoprocta

bryozoans (moss animáis)

Brachiopoda

lamp shells

Arthropoda

(Class Crustácea)

Ostracoda

ostracods

Copepoda

cyclopoids; harpacticoids

Tanaidacea

tanaids

Isopoda

isopods

Amphipoda

amphipods

Cirripedia

barnacles

Decapoda

crabs; lobsters; shrimp

Chordata

Ascidiacea

tunicates (sea squirts)

animáis in benthic communities are listed in Table 7.1 and illustrated in
Figures 7.5-7.7. They are described below, with particular attention being
directed to their positions in benthic food webs.

The best known of the benthic Protozoa are the foraminifera, whose
planktonic relatives were described in Section 4.2. Several thousand benthic
species are known, and they form a dominant element of the micro- and
meiobenthos, particularly in deep-sea sediments. Although they are
unicellular organisms, benthic forams are not necessarily small in size; some
attain lengths of 25 mm. There are both epifaunal and infaunal species, and
in general the various species feed on benthic diatoms and algal spores in
shallow water, and on other protozoa, detritus, and bacteria in all depth

184

zones. Newly discovered relatives, the Xenophyophoria (Figure 7.5a), are
especially abundant in hadal zones. They are the largest of all protozoans,
with diameters of up to 25 cm but only 1 mm thick. Their extended
pseudopodia (to 12 cm long) form tangled masses on the seafloor, and these
sticky structures probably collect organic matter from surface sediments.
Ciliates (Figure 7.4) are important members of the microbenthic community;
many are adapted to attach to sand grains or to live freely within the
interstitial spaces of sediments. The fragility of ciliates has hampered
sampling and ecological studies, but these protozoans are no doubt an
important link in shallow water between the microflora (e.g. benthic
diatoms) and larger animáis and, at all depths, between bacteria and
deposit-feeding invertebrates.

The most primitive multicellular animáis are the sponges (Figure 7.5b),
which may constitute a large fraction of the macrobenthos in some marine
regions. Known to exist from late Precambrian times (>600 million years
ago), this ancient group now has roughly 10000 species, almost all of them
marine. They are named for their porous nature (Phylum Porifera), and the
many cavities of sponges provide protective refuges for myriads of small
animáis such as worms and crustaceans. All sponges are sessile, that is, they
are attached and immobile. Most filter-feed by producing currents that draw

Figure 7.5 Representative benthic animáis:

(a) unicellular xenophyophore; (b) sponge;

(h) echiurid; (i) sipunculid; and
(j) pogonophoran. (All scales in mm.)

185

suspended partióles through the sponge. The pores of the sponge act like a
sieve, allowing only the smallest partióles to pass and be captured by special
flagellated cells. Food consists largely of bacteria, nanoplankton, and small
detrital partióles. The sponge skeleton is composed of calcium carbonate or
siliceous spicules embedded in the body wall, or of spongin fibers. Because
of their hard spicules, and reputedly because of their bad tasto, sponges have
few predators, the exceptions being some coral reef fish and some snails and
nudibranchs. They have both asexual and sexual reproduction, and are
capable of regenerating from only fragments of a whole organism.

The Phylum Cnidaria has many benthic representad ves in addition to the
pelagic species described in Chapter 4. This group too has had a long
evolutionary history, and there are now species living in most marine
environments. Most of the bottom-dwelling species are epifaunal but a few
exceptional species have adapted to live within sand or mud. Although there
is considerable diversity within the group, all benthic cnidarians are
characterized by having a radial symmetry, and all are suspensión feeders
that capture prey using nematocyst-laden tentacles. Some species also trap
very small particulate food in mucus secreted onto their oral surface. Benthic
cnidarians are sessile animáis, although some sea anemones are capable of
detaching from a substrate and swimming temporarily to escape starfish
predators. Asexual as well as sexual reproduction is common in
this phylum.

Within the Phylum Cnidaria, the Class Hydrozoa ineludes the colonial
hydroids (Figure 7.3), formed of unions of structurally and functionally
different types of individuáis. Although they are usually small and
inconspicuous, a large part of the marine growth attached to rocks, shells,
and wharf pilings and usually called ‘seaweed’ is actually composed of
hydrozoan colonies. Some hydroids produce free-swimming medusae as part
of their life cycle, but in the majority of species the medusa remains attached
to the parent, where it functions as a sexually reproducing individual. The
much larger Class Anthozoa, with over 6000 species, ineludes sea anemones
and a variety of coráis, as well as less familiar forms such as sea whips and
sea fans. Sea anemones (Figure 7.5c) are common residents of intertidal and
subtidal communities, but are also found at over 10000 m depth; they are
solitary animáis, ranging in diameter from about 1 cm to more than 1 m.
Included among the Anthozoa are a variety of taxonomically different forms
called ‘coráis’; the important subgroup of stony coráis that form massive
reefs in tropical regions is considered in more detail in Section 8.6.

Benthic worms belong to a number of different phyla. The threadlike
nematodes (Phylum Nematoda) constitute one of the most numerous and
widespread groups of marine (and terrestrial) animáis, although most of the
species are inconspicuous inhabitants of soft sediments. A single square
metre of bottom mud off the Dutch coast was reported to contain about
4 500000 individual meiobenthic nematodes. Taxonomic problems have
hampered ecological research on this abundant group, but it appears that
there is a wide diversity in feeding types with some species being
carnivorous, others feeding on plants, or on decaying material and associated
microfauna. The Phylum Nemertea (Figure 7.5d) encompasses about 600
species of elongated worms, all characterized by having a long eversible
proboscis that is used to capture food. Nemerteans are more abundant in
températe seas than in tropical areas, and they are more common in shallow
zones. Free-living flatworms (Phylum Platyhelminthes) (Figure 7.5f) reside

in sand or mud, under stones and shells, or on seaweeds, but they are seldom
present in large numbers. Sipunculids (Phylum Sipuncula) (Figure 7.5i),
also called peanut worms, are unsegmented worms ranging in length from
about 2 mm to more than 0.5 m. Many of the 250 or so species burrow into
sand or mud, using movements of their large proboscis to forcé their way
through the sediments; others inhabit rock or coral crevices, or even empty
snail shells. They are mostly deposit feeders. Echiurids (Phylum Echiura)
(Figure 7.5h) are somewhat similar to sipunculids in size and general habit.
Most species use their large nonretractible proboscis to forage for food
contained in sediments. Although some species occur intertidally, most are
found only in very deep water habitats. The majority of deep-sea echiurid
species have dwarf parasitic males attached to the female, a mode of
reproduction that is reminiscent of that of the deep-sea angler-fish (see
Section 6.6).

More than 10000 species belonging to the Phylum Annelida, Class
Polychaeta, make up the largest and most diverse group of marine worms.
Polychaetes (Figure 7.5e) are the segmented worms with múltiple
appendages called parapodia. Size ranges from a few millimetres to 3 m in
length. Ecoiogically, polychaetes can be separated into those that move
actively over the seafloor or burrow into sand and mud, and those that
inhabit permanent tubes or burrows. Most crawling species and some of the
active burrowers are camivorous and feed on various small invertebrates that
are captured with the predator’s jaws. Some polychaetes also use their jaws
to tear off pieces of algae. Many burrowers and some tube dwellers are
deposit feeders that consume sand or mud directly by mouth. Other
deposit-feeding species have developed special tentacle-like structures that
extend onto or into the substrate; sediment particles adhere to mucous
secretions on the surface of these structures, and this material is then
conveyed to the mouth by cilia. As well, many of the sedentary species are
filter feeders, using special head appendages to collect plankton and
suspended detritus. This group, with both epifaunal and infaunal species,
frequently forms a large fraction of the benthic biomass in many habitats.

The pogonophora (Figure 7.5j) are regarded as specialized annelids by
some workers, or as a sepárate phylum by others. These sessile worms are
most abundant in deeper areas, occurring down to 10000 m. They secrete
long leathery tubes that are attached to hard substrates. A cluster of tentacles
projects from the tube, and the common ñame of ‘beard worms’ is derived
from this feature. The pogonophora are highly unusual in lacking a mouth or
gut, but they share these features with the vestimentiferan worms that are
described in Section 8.9 and shown in Colour Píate 39. Both groups depend
on symbiotic chemosynthetic bacteria for their nutritional requirements,
although pogonophorans may also utilize dissolved organic substances
absorbed through their tentacles.

The Phylum Hemichordata ineludes the enteropneusts (Figure 7.5g), or
acorn worms, which occur intertidally as well as at deep-sea hydrothermal
vents (Section 8.9) and in trenches (Section 8.8). The largest species
reportedly attains lengths of over 1.5 m, but most are much smaller. Many
live in burrows in mud and sand, others move sluggishly over the sediment
surface, or form entanglements on firm substrates. Burrowing forms use their
proboscis to plough through the sediments, and most ingest sand or mud
from which the organic matter is digested. The amount of substrate
consumed is indicated by the large accumulations of faecal castings that

187

accumulate at the posterior opening of the burrow. Nonburrowing species
and some burrowers are suspensión feeders; plankton and detritus stick to
the mucus-covered proboscis and then are transponed in ciliated grooves to
the mouth. Enteropneusts are difficult to collect because of their fragility, but
deep-sea cameras have recorded their trails and tangled masses on the
seafloor.

QüESTION 7.2 Tab 1 e 7 J lists seven phyla o f d i f'fere n t i y pes of m ari ne worm s.
Whui features do these animáis share lo classify as a 'worm\ and why do
yon think they are such a sticcessful marine benthic group?

Members of the Phylum Mollusca inelude over 50000 marine species,
among them the familiar snails and related nudibranchs or sea slugs (Class
Gastropoda), and the bivalved clams and mussels (Class Bivalvia). This
phylum also ineludes the flattened chitons (Class Polyplacophora) (see
Figure 8.2), with a shell divided into eight plates. Less well-known members
are the burrowing scaphopods (Class Scaphopoda) (Figure 7.7a) with
tusk-shaped shells, and the wormlike, shell-less aplacophorans (Class
Aplacophora) found within sediments. Most of the octopus species (Class
Cephalopoda) are also essentially benthic species, although they are capable
of swimming. The great diversity in this phylum is expressed in the faets
that molluscs inhabit all depths of the sea, are found both on and within
sediments, have representative species in all trophic levels, and are present
in all benthic communities.

Echinoderms (Phylum Echinodermata) (Figure 7.6) are exclusively marine
animáis. Although differing in external appearance, all echinoderms are
characterized by having radial symmetry, with the body divided into five
parts around a central axis; a skeleton composed of calcareous plates; and
tube feet. The approximately 5600 species are divided into five classes. The
Class Asteroidea ineludes roughly 2000 species of starfish (or seastars) (see
Figure 8.2) whose habitats range from intertidal zones to about 7000 m.
Many starfish are carnivorous, and they may have considerable ecological
impact on cultivated shellfish beds as well as in natural habitats. Other
starfish species are deposit feeders or, more rarely, suspensión feeders. The
Class Ophiuroidea comprises almost 2000 species of long-armed brittle
stars (Figure 7.6c) and basket stars. Deep-sea photographs often show
ophiuroids carpeting the seafloor, where they feed on deposited sediments,
on small dead or living animáis, or on suspended organic material. Some
800 species of spiny sea urchins (Figure 7.6d) and flattened sand dollars are
placed in the Class Echinoidea. Urchins are conspicuous members of the
macrobenthos of rocky shores, kelp beds, and coral reefs; they use a special
chewing apparatus to feed on all types of organic material, but most
shallow-water species are regarded as basically herbivorous and deep-sea
species (to about 7000 m) are considered to be deposit feeders. Sand dollars
are inconspicuous, but often numerous, infaunal species; some are capable of
suspensión feeding as well as deposit feeding. The Class Holothuroidea
(with 500 species) ineludes the elongated sea cucumbers (Figure 7.6b),
named because of their resemblance to the vegetable. The epibenthic species
of holothurians can be either deposit or suspensión feeders; infaunal species
swallow the sand or mud in which they live. Although also found in shallow
waters, the greatest number of abyssal echinoderms are sea cucumbers, and
they feature prominently in deep-sea photographs. The Class Crinoidea is
the most ancient echinoderm group, and presently ineludes about 650 species
of animáis known commonly as feather stars and sea filies. Feather stars

188

Figure 7.6 Representative benthic echinoderms: (a) feather star (crinoid); (b) sea cucumber
(holothurian); (c) brittle star (ophiuroid); and (d) sea urchin (echinoid). (All scales in mm.)

(Figure 7.6a) live mostly in depths above 1500 m, and although they often
cling to the seafloor, they are mobile animáis that are capable of crawling as
well as of swimming temporarily. Sea lilies are attached by stalks to the
seabed, and they are typically deep-sea inhabitants that are most abundant
between 3000 m and 6000 m. All crinoids are considered to be suspensión
feeders.

Bryozoa (Figure 7.7b), or moss animáis, belong to the Phylum Ectoprocta.
Like the hydroids, bryozoans are colonial and sessile animáis that form
inconspicuous encrustations or seaweed-like growths on intertidal rocks,
shells, or artificial surfaces. A few species have also been recorded from
depths of more than 8000 m. Each individual in a colony is small (usually
<0.5 mm long) and, in the majority of species, is largely encased in an
extemal calcium carbonate skeleton. A special food-trapping device, called a
lophophore, consists of numerous ciliated tentacles, and it can be projected
into the overlying water to capture small planktonic food or suspended
detritus. Despite including almost 4000 marine species, the group has
received little attention from ecologists.

Brachiopods (Figure 7.7c) constitute a phylum (Brachiopoda) of somewhat
less than 300 marine species that superficially resemble molluscs in having a

189

Figure 7.7 Representative benthic animáis:

(a) scaphopod; (b) bryozoan colony; (c) stalked
brachiopod; (d) two tunicates; (e) tanaid;

(f) isopod; and (g) a horseshoe crab (not
related to true crabs). (Al! scales in mm.)

bivalved calcareous shell (5-80 mm in diameter), though their fundamental
body-plan is quite different. Most live in the upper 200 m, and most are
cemented to a hard substrate. However, some of the more common
brachiopods (e.g. Ungula) live in vertical burrows in sand or mud, and some
have been collected from 5500 m. Like the bryozoans, brachiopods possess a
lophophore that is employed in suspensión feeding.

Tunicates (or ascidians) (Figure 7.7d) are benthic relatives of the pelagic
larvaceans and salps, described in Section 4.2. These sessile barrel-shaped
animáis belong to the Phylum Chordata, Ciass Ascidiacea. Most of the
common tunicates are solitary organisms, but there are also many colonial
species that develop by asexual budding. Ascidians are commonly found in
intertidal waters, attached to rocks, shells, wharves, or other firm substrates,
but they also inhabit depths to at least 8000 m. The free end of a tunicate
has two siphons that provide passage for a current of water drawn through
the animal by cilia. Suspended partióles are removed from the water by a
sheet of secreted mucus, and entangled food is conveyed to the gut by cilia.
Filtered water is forcibly expelled from the animal’s excurrent siphon, giving
rise to the common ñame of ‘sea squirtk Deep-sea forms exhibit
modifications in the feeding apparatus, and they are thought to feed on
suspended sediments, or even directly on meiobenthos. In general, however,
the suspension-feeding mechanism of these benthic tunicates is similar to
that employed by their pelagic relatives, the salps. Although ascidians are
generally not major elements of benthic communities, they are capable of

190

removing significant amounts of plankton or suspended material from water
in proximity to the seafloor; a single tunicate only a few centimetres long
can filter about 170 litres of water daily.

The segmented Crustácea are well represented on the seafloor. Meiobenthic
species inelude ostracods and the cyclopoid and harpacticoid copepods, all
of which were described briefly in Section 4.2. Harpacticoids are an
especially abundant group whose members crawl over or burrow through
soft sediments. Also included in this size category are the tanaids
(Figure 7.7e). These small (usually <2 mm long) crustaceans have slender,
more or less cylindrical bodies, and they are burrowers or tube dwellers
present at all depth levels down to at least 8000 m. Little is known about the
biology of these 350 species.

Common macrobenthic crustaceans inelude the isopods (Figure 7.7f) and
amphipods. Isopods usually have a flattened body of 5-15 mm in length;
deep-sea species are generally larger, however, with one genus reaching
40 cm. Isopods are often observed running rapidly over rocks in the
intertidal zone, but some species are burrowers, including one group that
tunnels into wood. The majority of the 4000 marine species are omnivorous
scavengers. Amphipods are closely related to isopods, but differ in that most
have a laterally compressed body (see Figure 4.10 k for planktonic species).
They range from a few mm to about 30 cm in size, with the largest species
occurring in the deep sea. Depending on the species, amphipods are capable
of crawling or burrowing, but many of the bottom-dwellers are also capable
of swimming, even if infrequently. Many of the species construct temporary
or permanent burrows or tubes. The depth distribution in this group ranges
from the semi-terrestrial beach fleas living near the high tide level to species
living in hadal trenches. Most amphipods are detritivores or scavengers, but
a few are specialized filter feeders.

Barnacles (Cirripedia) (see Colour Píate 31) are familiar marine animáis and
the only sessile crustaceans. There are about 800 species, including a large
number that are parasitic in other marine invertebrates. These shrimplike
animáis live within an extemal covering of calcareous plates. Some attach
directly to substrates, others are stalked. The more familiar barnacles form
crowded aggregations in rocky intertidal regions, but some species have
become specially adapted to attach to mobile surfaces and live on the bodies
of whales, sharks, sea snakes, manatees, fish, or crabs. Although most
common in shallower depths, some species are present to at least 7000 m.
Free-living (i.e. nonparasitic) barnacles feed by rhythmically sweeping their
feathery appendages through the surrounding water. These animáis often foul
the bottoms of ships and the surfaces of buoys and wharf pilings. They have
been the subjeet of extensive ecological studies since Charles Darwin’s
elassie taxonomic monograph on this group.

Benthic decapod crustaceans inelude the familiar crabs, lobsters, and
shrimp, and the group has both epifaunal and infaunal representatives.
Decapods show their greatest diversity in shallower waters, but a few species
live at depths of 5000-6000 m. The group ineludes predators, omnivores,
and scavengers. Some are filter feeders (e.g. burrowing mud shrimp and
mole crabs), but detritus rather than plankton is often the dominant food.
Many species in this group are economically important as human food, and
those species along with molluscs make up the shellfish industry.

191

There are a few other phyla of marine animáis that have not been considered
here, either because they consist of only a few species or because they are
not usually abundant in benthic communities. In some cases, we still lack an
understanding of the ecological role of certain obscure animal groups simply
because present sampling gear does not adequately collect them. Many new
species (as well as new families) have been discovered within the last
25 years, primarily in the deep sea (see Sections 8.8 and 8.9), and this list
can be expected to continué to grow.

QUE5TIQN 7.3 From Information in the previous section* what are some of
the mechan i sois used by benthic animáis for defence against preda t ion?

7.2.2 SAMPLING AND PRODUCTION MEASUREMENTS

Benthic animáis can be collected by a variety of sampling gear. In shallow
waters less than about 30 m, direct observation, counting, and collection by
scuba diving are effective techniques that permit a diver to sample any type
of substrate and provide a greater appreciation of the natural conditions,
including patchy distribution of the macrobenthos. In depths beyond safe
diving limits, grabs or box corers are commonly used to sample soft
sediment communities. Ideally, these devices remove a quantitative bottom
sample from which the resident animáis can be screened out on a series of
sieves. Hydraulic cores of undisturbed samples can be obtained but because
these are small, they are used only for quantitative sampling of meio- or
microbenthos. Dredges are containers equipped with sturdy bags of fairly
wide mesh that are pulled over the seafloor; they are designed to scoop up
benthic animáis, but they do not generally provide quantitative samples.
Other types of sampling devices are mentioned in Chapter 8 in connection
with special environments.

Standing stock of benthos is given in terms of numbers of animáis per unit
area (usually m^ 2 ). Benthic biomass is expressed in terms of g m -2 ; ideally,
these data are presented in grammes of carbón or grammes of dry weight,
but they may be given in grammes of wet weight (for conversions, see
Appendix 2). Biomass changes constantly, and measures of secondary
productivity take this into account. Biomass increases through growth and
reproduction of the individuáis in a population or community; existing
biomass is removed by predation or other sources of mortality. Under certain
circumstances, these changes can be monitored over successive intervals, and
production can be determined from the methods given in Section 5.3.1 (see
especially equations 5.3, 5.4, and 5.5).

Age classes may be distinguishable in populations that breed during a
well-defined time period, or from age-size relationships, or in species that
have age-related features such as growth rings on shells. If the biomass of
each age class is determined and changes in biomass are followed by
successive sampling, it is possible to calcúlate production during the interval
between samples. Table 7.2 presents production data collected over a period
of 616 days from a population of infaunal clams in the North Sea. In the
first interval of 50 days, mortality was very high but the mean weight of the
surviving individuáis increased by about 4 mg; this resulted in a decrease in
biomass, but a net production of 317 mg m -2 day~ l . Completing the
calculations in the table will show that proportionally more biomass was
produced, but then lost from the population due to predation or other
sources, during the first two intervals than later in the study.

192

Table 7.2 Production data from a population of the clam Mactra in the North Sea.

Mean

Biomass

Net

weight

Biomass

loss

production

(days)

(no. m -2 )

(mg)

(mg m -2 )

(mg m -2 day” 1 )

(mg m -2 day -1 )

7045

1.416

9976

411

317

990

5.364

5310

225

378

9.910

398

289

44.286

616

246

73.542

QUESTION 7,4 ín Table 7.2. biomass has becn calculated only tbr the lirst two
age classes. and loss of biomass and net production only for the first interval
of 50 days. Complete the rest of the table using equation 5,3 to calcúlate
biomass. and equation 5.4 to calcúlate biomass losses and net produclUm.
(Refer to Section 5.3.1 for help if necessary.)

If it is not possible to follow a single cohort of animáis and there are
numerous overlapping age classes, benthic secondary productivity becomes
more difficult to assess directly. In such situations, laboratory studies can be
used, if feasible, to determine typical growth rates of each arbitrary size
class. The population is then divided into component size classes regardless
of age, and production is calculated by multiplying the numbers present in
each size class by the growth rate of that class.

7.3 DETERMINANTS OF BENTHIC COMMUNITY
STRUCTURE

The numbers and types of organisms making up any particular benthic
community are determined by a variety of physical and biological factors.
Among physical factors, sediment type is of paramount importance in
dictating the type of community that can become established in a particular
locality. The relative proportions of epifauna and infauna will vary according
to whether the seafloor surface is composed of rock, mud, sand, or some
combination of these types. At the same time, however, the activities of the
organisms themselves will alter the substrate. Burrowing and feeding
activities of the infauna continually disturb and rework soft sediments (a
process called bioturbation), and sessile organisms that attach to hard
surfaces alter the topography of their environment. In shallow Coastal waters,
tidal levels, degree of wave action, and salinity and temperature variations
will influence the composition and relative abundance of benthic species.

Biological determinants of benthic community structure inelude competition,
predation, and the type of development leading to recruitment into the adult
population. As in pelagic communities (Section 5.2), competition among
individuáis or among species for limited resources (e.g. food) may affect
population densities and relative abundance of species. Competition for
limited space in hard-substrate communities may influence the distribution
pattems of sessile zoobenthos. Selective feeding by predators can alter the
outeome of competition between prey species, and predation may affect total
species diversity in benthic communities. The effeets of competition and

193

predation in specific types of communities are discussed in more detail in
Chapter 8.

Community structure also can be influenced by the type of development that
is exhibited by dominant species. Benthic animáis may have direct
development, in which there is no free-swimming larval stage and the young
emerge as miniature adults, or more commonly, they may have indirect
development with the production of meroplanktonic larvae (see Section 4.3).
Two types of meroplanktonic larvae are distinguished. Lecithotrophic
larvae hatch from relatively large eggs that are produced in small numbers
by the parent. They spend a very brief time in the water column (hours to
2 weeks) and usually do not feed on planktonic food, depending instead on
yolk in the egg for their growth and development. Planktotrophic larvae
hatch from small eggs produced in large numbers; they rely on
phytoplankton or bacteria for nourishment and remain in the pelagic
environment for several weeks or months.

Lecithotrophic larvae, which spend a very short duration in the plankton, do
not usually drift far from the adult population which produced them, but
planktotrophic larvae remain in the water column for longer times and can
be transported by currents over long distances. Consequently, planktotrophic
larvae in particular are a means of dispersal for species that are immobile or
slow-moving as adults. Both larval types are vulnerable to pelagic predators
while they are planktonic, and planktotrophic larvae especially may be
carried away by currents from suitable benthic substrates upon which they
could successfully settle. Lack of abundant planktonic food may also
contribute to the higher mortality rates of planktotrophic larvae. As
meroplanktonic larvae develop and grow, they become photonegative and
begin to move deeper in the water column, toward the seafloor. Many, if not
all, pelagic larvae actively search for a suitable substrate before selecting one
on which to settle and metamorphose into the adult form. Larvae seeking
settlement sites rely on a variety of cues such as substrate type, suitable
food, light intensity and, in many cases, on the presence of adults of their
own species.

The relative success of larval recruitment into adult populations will
influence community structure in terms of relative numbers and biomass of
the resident species. Species that produce planktotrophic larvae often have
variable recruitment success; in successful years, many larvae may survive to
settle in the adult population, but in other years, larval mortality may be high
and recruitment low. In comparison, populations of species with
lecithotrophic larvae (or those with direct development) tend to have low but
constant biomass over long periods.

QUESTiON 7.5 Benthic in vertebrales with direct development (no pelagic
larval stage) may neveitheless be widely distributed, Can you think of any
way(s) in which such species could disperse to new geographic local i lies?

7.4 SUMMARY OF CHAPTER 7

1 The benthic environment is divided into a number of distinctive
ecological zones based on depth, seafloor topography, and vertical gradients
of physical parameters. These are the supralittoral, littoral, sublittoral,
bathyal, abyssal, and hadal zones.

2 Benthic plants inelude macrophytic angiosperms like mangrove trees,
marshgrasses, and seagrasses. Macrophytic algae inelude green, red, and
brown seaweeds, and the long-stemmed kelps, a type of brown algae.
Microphytic algae inelude benthic species of diatoms, Cyanobacteria, and
dinoflagellates.

3 The number of phyla and the number of species of benthic animáis
exceeds those of pelagic species, at least partly because of the greater
physical variety of benthic habitats.

4 Benthic animáis are separated into infaunal and epifaunal species,
depending upon whether they live within sediments or on the surface of the
seafloor, respectively. Size categories of the zoobenthos consist of the larger
macrofauna (>1.0 mm), the small meiofauna which is characteristically
found in sand and mud, and the microfauna which is made up mostly of
protozoans.

5 Benthic primary productivity is measured by a variety of methods
including the carbón-14 method for microphytic species, and harvesting and
weight measurements of macrophytic plants. Methods of estimating benthic
secondary production are similar to those employed for pelagic animáis and
described in Chapter 5.

6 The numbers and types of species making up any particular benthic
community are determined by a variety of physical and biological factors. In
shallow Coastal communities, the types of species present and their relative
abundance will be partly determined by tidal levels and degree of exposure
to air, wave action, and range of salinity and temperature. At all depths, the
type of sediment (e.g. sand, rock, mud) will dictate the relative proportions
of epifauna and infauna. Biological factors that influence benthic community
structure inelude competition for limited resources (e.g. food, space),
predation, and type of development.

7 Benthic animáis may have direct development, in which there is no
free-swimming larval stage, or they may produce pelagic planktotrophic or
lecithotrophic larvae. Planktotrophic larvae are relatively small and are
produced in large numbers; they must feed on planktonic food, and they
remain in the water column for several weeks or months. Although they are
a means of dispersal for the species, planktotrophic larvae have high rates of
mortality, and the adult populations have variable rates of recruitment from
year to year. Lecithotrophic larvae hatch from relatively large eggs that
contain large amounts of nutritive material and that are produced in small
numbers; these larvae do not remain planktonic for long, and they do not
feed while in the water column. Compared with planktotrophic larvae,
mortality rates are lower for lecithotrophic larvae, and species with this type
of development tend to have low but constant biomass because recruitment
is less variable.

Now try the following questions to consolídate your understanding of this
Chapter .

QUESTIQN 7,6 Compare Tabíe 7.1 with Table 4J (a) How many phvlu are
represented in the plankton and in the bembos? (b) Why is there a difference
in the numbers oí phyla in these major marine envrronmems?

195

QUESTIOH 7.7 Wüuld you cortsider benthic species tbat produce leeithotrophi
larvae to ha ve predominantly r- or K -selection characteristícs? Explain the
bases for your answer (Refer to Section 1.3.1 and Table l.L)

0UEST10N 7,8 Slromatolites (see Section 7 A) in Hainelin Pool. Western
Australia, reach a máximum height of 1.5 m. Assuming no erosión or
change in sea level, how oíd are diese structures?

CHAPTER 8

BENTHIC COMMUMTIES

This chapter considers different types of benthic communities, ranging from
the highest intertidal levels to the deepest trenches. Although these benthic
habitats are treated separately here, they are all coupled in a dynamic fashion
with the overlying pelagic environment.

In shallow water, both phytoplankton and benthic plants contribute to the
primary production of benthic communities. Phytoplankton and zooplankton
are consumed by shallow-living filter feeders. Conversely, exudates and
detritus from attached benthic plants supply necessary nutrients for the
phytoplankton and planktonic bacteria, and bottom currents may cause
resuspension of sediments, making benthic microphytes and bacteria-covered
sediment particles available as food sources for zooplankton. Some fish and
some marine mammals rely on shallow-water benthos for food.

The great majority of benthic communities, however, are located in the
aphotic zone, and most are entirely dependent on organic matter that is
photosynthetically produced in the euphotic pelagic zone. The only
exceptions are certain communities in which the food chain begins with
chemosynthetic production by bacteria (see Section 8.9). Part of the organic
matter that sinks or is transported from the surface waters is the food source
that supplies deep-water benthic communities. Sinking organic and inorganic
particles also form the sediments in which the benthos live. Decomposition
processes tend to take place in deep water or on the seafloor, and the
nutrients that are released are eventually returned to the surface where they
are used by the phytoplankton.

Marine organisms may exploit both the benthic and pelagic environments
during different stages of their Uves. Many invertebrates are benthic as
adults, but disperse by producing planktonic larvae. Conversely, some
planktonic organisms produce resting stages (spores, cysts, or eggs) that sink
into sediments where they remain dormant until favourable water conditions
cause them to ‘hatch’ into swimming or drifting stages.

The concept of benthic-pelagic coupling recognizes the many interactions
between these two vast environments, and attempts to intégrate the ecologies
of the seafloor and the water column.

8.1 INTERTIDAL ENVIRONMENTS

The terms littoral and intertidal are synonyms for that Coastal area which is
periodically exposed to air by falling tides and submerged by rising water
levels. Included in this general area are a variety of distinctive ecosystems
such as rocky shores, sandy beaches, and mudflats, each supporting specially
adapted assemblages of species. Rocky intertidal areas support a
preponderance of epifauna, whereas the soft-substrate sand and mud
communities have higher proportions of infaunal species. The intertidal
regions mark the transition from land to sea, and although they make up
only a very small part of the total world ocean, they support rich and diverse
communities of marine plants and invertebrates as well as birds and inshore
species of fish. Even some land mammals (e.g. mink, skunks, and raccoons)

197

visit this area to feed on easily accessible shellfish, and many shorebirds
depend on the rich food supply to be found in these habitats.

8.1.1 TIDES

Tides are the periodic rise and fall of sea level over a given time interval,
and they are caused by the interaction between the gravitational attraction of
the Moon and Sun on the Earth, and the centrifugal forcé resulting from the
rotation of the Earth and Moon. On most coasts, semidiurnal tides result in
the intertidal zone being exposed and covered by water twice each day.
However, because of certain physical conditions, there is only one tide per
day (a diurnal tide) in some regions such as the Gulf of México.

Tidal range is greatest during spring tides which occur twice each month
when the Earth, Sun, and Moon are aligned. At the other extreme, tidal
range is minimal during the neap tides which occur at the first and the third
quarters of the Moon, when these planetary bodies are not in alignment. The
high water mark is the greatest height to which the tide rises on any day,
and the low water mark refers to the lowest point to which the tide drops.

The extent of the littoral zone in any particular locality is governed by the
slope of the shoreline and by local tidal ranges, which are partly determined
by the configuration of coastlines. Tidal range varies from barely perceptible
in places such as Tahiti and the Baltic Sea, to as much as 15 m in the Bay of
Fundy in eastem Cañada.

8.1.2 ENVIRONMENTAL CONDITIONS AND ADAPTATIONS OF INTERTIDAL
ORGANISMS

The littoral regions experience the greatest variations in environmental
conditions of any marine areas. Here, organisms are periodically exposed to
air, and they encounter wide fluctuations in temperature and salinity. Rainfall
and land runoff both contribute to lowering salinity. In coid climates,
intertidal organisms are subjected to ice formation and ice scouring. In
addition, many intertidal regions are exposed to heavy wave action and
current motion.

Intertidal plants and animáis show a variety of special adaptations to the
changing conditions of their environment. Whereas inhabitants of sand and
mud tend to burrow into the soft substrates to escape desiccation,
temperature and salinity extremes, and wave action, organisms living on
rocky shores exhibit more diverse adaptations to these environmental
features. Rocky-shore species of bivalved molluscs (e.g. clams, mussels) and
barnacles cióse their shells tightly during emersión, enabling them to retain
moisture around the gills and thus preventing desiccation as well as exposure
to freshwater. Many snails also retreat into their shells, sealing the shell
aperture with a horny or calcareous operculum on the foot. On the other
hand, many benthic plants and some of the intertidal animáis have no
particular mechanisms to avoid water loss. Algae like Fucus and
Enteromorpha , for example, tolérate as much as 60-90% loss of water from
their tissues.

Shells, or other types of rigid exoskeletons like those of sea urchins, also
protect animáis from mechanical injury in areas where wave action can be
severe. In some sea urchins and molluscs, the shell is much thicker in
populations exposed to heavy wave action than in populations which are

sheltered. Strong attachment to rock surfaces or other firm substrates
prevenís plañís and animáis from being washed away by waves and currents.
Beníhic algae aííach lo rocks by special holdfasls. Barnacles, oyslers, some
lubeworms, and lunicaíes secrele cemeníing subslances for firm attachment.
Mussels secrele lough elaslic byssal íhreads from a special gland in íhe foot,
and íhese secure íheir posilions. The broad, flallened fool of limpets,
abalones and chitons provides a suction-like attachment, and their low,
streamlined profiles also help to resist wave impact. Certain animáis (e.g.
some sea urchins and rock-boring clams) are equipped to bore into hard
surfaces by mechanical abrasión, Chemical secretions, or both. Many of the
more mobile intertidal animáis, like crabs and isopodes, seek out rock
crevices where the wave action is reduced; this sheltering behaviour also
permits them to remain in moist refuges at low tide. Rock pools form similar
refuges for animáis such as starfish, crabs, and small intertidal fish, all of
which avoid desiccation by remaining in these pools at low tide.

8.2 ROCKY INTERTIDAL SHORES

Much is known about the inhabitants and ecology of rocky shores compared
with other marine habitats. The accessibility of these densely-populated
marine communities has permitted researchers to make long-term direct
observations, and to conduct in situ experiments on factors determining
community structure.

8.2.1 ZONATION

A striking characteristic of all rocky shores is that the resident plants and
animáis are grouped in distinctive bands, with some species living high in
the intertidal zone and others being grouped at lower tidal levels. This
vertical zonation of species applies to all rocky intertidal communities,
although the specific pattern of zonation and the species composition of the
zones varíes according to geographic location, tidal range, and whether sites
are exposed to severe wave action or are protected. Zonation is largely based
on sessile species, like algae, barnacles and mussels, although some mobile
animáis also tend to be zoned but with less sharp demarcation. In general,
many of the larger motile animáis move with the tides and often remain in a
relatively constant water depth, or retreat to rock pools at low tide.

On rocky shores, the supralittoral zone (see Chapter 7) is inhabited by
encrusting black lichens (which are combinations of algae and fungi) and
blue-green algae, certain species of Littorina (periwinkles) that graze on the
vegetation, and relatively large (3-4 cm long) isopods {Ligia). Primitive
insects (e.g. Machilis) may also be present.

Just below the supralittoral zone, periwinkles (.Littorina species) are usually
found in extraordinarily dense populations, with numbers ranging from
several hundred to 10000 snails per square metre. Lower in the intertidal,
barnacles form a sharply demarcated belt, and these crustaceans may also
have densities of thousands per m 2 . In many localities, mussels crowd
together in dense aggregations below the barnacle zone. There is intense
competition for the limited space among the attached algae and sessile
animáis as shown in Colour Píate 31.

199

Gregariousness is an adaptive feature in many of these intertidal species. By
crowding together, periwinkles create microhabitats in which more moisture
is retained during exposure at low tides. They reproduce by internal
fertilization, and crowding also increases their chances of finding mates.
Mussels freely release gametes into the sea, where fertilization occurs; in
this case, gregariousness increases synchrony of spawning among many
individuáis. Barnacles are hermaphrodites that cross-fertilize, and high
population densities are necessary for reproduction. The penis of a barnacle
can only reach about twice as far as the diameter of its exoskeleton, so it is
essential that the animáis be in cióse proximity.

Intertidal zonation of organisms is not determined simply by tidal levels, but
results from a variety of physical and biological causes. The upper limit of a
particular zone is often determined by physical factors and the ability of
particular plants and animáis to deal with exposure to air and with
temperature and salinity variability. The upper limit of any one species may
also be set by biological factors such as the absence of suitable food, or
grazing or predation pressure. The lower limit of a particular zone is usually
determined only by biological factors.

How can the causes of vertical zonation be determined?

Sessile animáis, like barnacles and mussels, make ideal subjects for studies
of vertical zonation because the same individuáis can be monitored over
long periods. Photographs taken at successive intervals record the size and
position of individuáis and enable researchers to follow growth, interactions
with neighbouring individuáis, and death. The accessibility of the intertidal
zone also makes experimental manipulation possible, and individual animáis
may be removed from sites, or whole rocks with attached organisms may be
relocated to different tidal levels. It is also possible to exelude predators by
enclosing study populations within wire cages. The results of such
techniques are described below.

Barnacle populations on rocky shores in Scotland are composed of two
species (Figure 8.1). Adults of the small Chthamalus montagui form a
distinct band high on the shore, above the mean high water mark of neap
tides, with only a few adults being found down to the mean tide level. The
larger Balanus balanoides has a much wider distribution for both adults and
larvae, extending above and below the mean neap tide marks. Long-term
observations indicated that the larvae of Chthamalus actually settle
throughout most of the Balanus zone, but only survive to adulthood in the
upper areas. This is because young Chthamalus are eliminated from areas
below the mean high water neap mark by competition for restricted
attachment space with the faster growing Balanus . Balanus either overgrows,
undercuts, or crushes Chthamalus. The observation that the lower limits of
Chthamalus zonation are dictated by biological competition was confirmed
by experimental work in which all newly settled Balanus larvae were
removed from rocks containing young Chthamalus ; in such studies,
Chthamalus survived well at all tidal levels. On the other hand, the upper
zonation levels for both barnacle species are determined by physical factors,
with Chthamalus being more tolerant to heat and/or desiccation than
Balanus . Further, intraspecific competition may be an important mortality
factor for Balanus ; if larval recruitment is high, the growing barnacles begin
to compete for space, and the younger or slower-growing individuáis of the
species are eliminated.

200

Balanus óatanoiúes

Chtfiamtus montagtsi

mear high
water spring

mean higfi
water neap

mean tide
leve!

mean low
water neap

mean low
water spring

aduft larval mortality
distribuí ron tacto rs

adule larval
dlstrlbution

mortality
laclors

Figure 8.1 The effeets of competition and predation on barnacle distribution in Scotland.
C u intraspecific competition; C 2f interspecific competition between Chthamalus and Balanus,
D, desiccation; P, predation by Nucella , a predatory snail. The widths of the distribution bars
indícate relative abundance; the widths of the mortality bars indícate relative importance of
the factors concerned. Note that the upper limits of distribution for both species are
determined by physical factors (i.e. tolerance to dessication). Snail predation and
intraspecific competition for space are the major causes of mortality for Balanus, and both
factors become increasingly important at lower tidal levels. For Chthamalus, the major cause
of mortality of newly settled larvae is intraspecific competition for space with the
faster-growing Balanus . Few Chthamalus larvae settle below mean tide level, but those that
do are eliminated by predation and interspecific competition.

Predation too may be an important determinant of zonation pattems. The
whelk Nucella lapillus is a major predator of bamacles in Scotland. Like
many predators, this snail prefers to eat larger prey, and thus prefers Balanus
balanoides over Chthamalus. When cages were used to exelude Nucella
from natural populations of barnacles, it became evident that snail predation
was a major cause of mortality for larger (and older) Balanus , especially
those living in the lower intertidal zone where Nucella was most abundant.
Thus the lower limit of the Balanus zone is determined largely by predation.

Similar predator-prey relationships can be found on the west coast of North
America between three barnacle species ( Chthamalus dalli , Balanus glándula
and B. cariosas) that are preyed upon by three different species of Nucella.
In this area, most of the mortality of young Balanus glándula is the result of
predation rather than crowding and competition for space, and B. cariosus
attains an adult size that is too great to be eaten by Nucella. However, here
too predation and competition for space act to set the lower limits on the
barnacle distributions.

Physical and biological factors also act in concert or independently to set the
zonation patterns of benthic algae. Algae compete for sunlight (see
Section 7.1) and for restricted space with other plants and with animáis, and
these biological factors partly establish the position of plants on shores.
Upper zonation limits of algal species are often set by tolerances for
exposure and desiccation, and may also be determined by the grazing
pressure of herbivores. As an example, the Torrey Canyon oil spill in 1967
killed the dominant grazing molluscs in the intertidal areas of parís of
south-western England, and subsequently there was a rise in the upper
zonation limits of several intertidal algae. With eventual recovery of the
grazers, the higher reaches of the algae were once again grazed down and
the original zonation pattern was re-established.

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8.2.2 TROPHIC RELATIONS AND THE ROLE OF GRAZING AND PREDATION IN
DETERMINING COMMUNITY STRUCTURE

Both benthic algae and phytoplankton are important primary producers
supporting rocky intertidal communities, but production figures are relatively
low. The intertidal zone is a difficult habitat for benthic algae in several
respects. In tropical regions, heavy rainfall, high light intensities, and
exposure to high air temperatures with resulting desiccation are major
problems. Freezing and scouring by ice limit algal production in arctic and
subarctic intertidal areas. In températe climates, where benthic algae reach
their full potential, there is competition between algal species for access to
sunlight, and competition for attachment sites with other algae and with
sessile animáis. The average annual productivity of the rocky intertidal areas
of the world is of the order of 100 g C m~ 2 . However, production rates of
around 1000 g C m~ 2 yr^ 1 may occur in particularly favourable areas.

Attached algae are grazed by a variety of molluscs and sea urchins. Mussels,
barnacles, clams, tunicates, tubeworms (polychaetes), and sponges are
among the many filter feeders that are dependent on plankton. Intertidal
carnivores inelude starfish, which eat limpets, snails, barnacles, mussels, and
oysters; predatory snails, which consume a variety of prey including clams,
mussels, and barnacles; and sea anemones, whose prey ineludes shrimp,
small fish, and worms. Important scavengers inelude isopods and crabs.
Shorebirds may also have considerable predation impaets on intertidal life
(see Sections 6.5 and 8.5).

Experimental work has demonstrated that herbivores such as sea urchins,
limpets, chitons, and littorine snails may control both the level of primary
production and the species composition of benthic plants. For example,
removal of limpets from experimental areas results in the appearance of
different species of algae and in heavier algal growth compared with
undisturbed sites. Removal of sea urchins from intertidal and subtidal
regions also tends to create a greater initial diversity of algal species,
although this may eventually change to lower diversity as other determinants
of community structure come into play. The species composition of the algae
in a community may also result from competition for space and light
between different algal species, with the dominant species being those that
are fastest growing in the particular locality.

Competition and predation are also important determinants of species
composition and diversity among intertidal animáis. Along the north-west
Pacific coast of North America, the rocky intertidal community is dominated
by mussels, barnacles, and the carnivorous starfish Pisaster ochraceus.
Pisaster feeds on a variety of molluscs and barnacles, as illustrated in
Figure 8.2. Experimental removal of the starfish from the community
resulted in lowering species diversity from about 30 species to one dominant
species, the mus sel Mytilus califo miarais. When Pisaster is present, its
feeding activities control the numbers of dominant sessile prey (barnacles
and mussels) so that space is kept open and none of these species becomes
dominant; at the same time, primary production is also enhanced by the
provisión of space for benthic algae. When the top predator is removed,
competition for space is intensified and, in this región, Mytilus californianus
overgrows and outeompetes all other macrobenthos to take over the available
space throughout most of the mid-intertidal zone. Pisaster is referred to as a
keystone species because its activities disproportionately affect the patterns
of species occurrence, distribution and density in the communities in which

202

Figure 8.2 The food of the starfish Pisaster
ochraceus, a keystone species of rocky intertidal
communities along the Pacific coast of North
America. Numbers in parentheses indícate the
number of species in a particular group.
(Animáis are not to scale.)

it lives. Similar types of interactions control rocky intertidal community
structure in other areas of the world. In New England, for example, the
mussel Mytilus edulis is the competitively dominant sessile species, whose
numbers are usually kept in check by two species of starfish ( Asterias
forbesi and A. vulgaris) and by the snail Nucella lapillus .

QUESTIOM 8.1 Would you expect to find a greater biomass per unit area of
benthic organisms in intertidal areas with a high tidal ranga {e.g. >2 m). or
in those with a small tidal range (e.g, <0,5 m), and why?

8.3 KELP FORESTS

ln coid températe regions, intertidal rocky-shore communities merge
subtidally into kelp forests. The term kelp refers to a variety of very large
brown algae that are usually found only outside the limits of the 20°C
summer isotherms. These algae form distinctive subtidal communities in
areas of upwelling, fast currents, or heavy surf. Kelp require a hard substrate
for attachment, and they grow off rocky shores to depths of 20-40 m,
depending on water clarity. Kelp beds occur along the western coasts of
North and South America, extending into subtropical latitudes in upwelling
areas. In the western Pacific, extensive kelp beds are found off Japan,
northem China, and Korea. In the Atlantic, large kelp beds occur off the
Canadian east coast, and off the coasts of Southern Greenland, Iceland, and
northem Europe including the United Kingdom. The highest biomass levels
of kelp are around subantarctic islands like the Falklands. New Zealand and
South Africa are also localities that support kelp in quantities sufficient for
exploitation.

Each kelp plant typically has a holdfast for attachment to the substrate and a
flexible stipe (or stalk) (Figure 8.3). Large, thin blades (equivalent to leaves)
are attached to the stipe. Kelp may also have gas-filled floats (or
pneumatocysts) which keep the blades near the water surface where solar
radiation is highest. Because of their large photosynthetic surface and the
constant supply of nutrients in the surrounding turbulent water, kelp are
highly productive plants. Common Pacific genera inelude Nereocystis,
Postelsia , and Macrocystis. Macrocystis pyrifera is commonly known as the

203

Figure 8.3 A number of kelp species
illustrating diversity of structure in this group of
brown algae. (a) Nereocystis luetkeana,

(b) Poste/sia palmaeformis, (c) Laminaria
saccharina

giant kelp as it may exceed 50 m in length, and it forms aquatic forests off
California. Various species of Laminaria (usually 3-5 m in length) are the
dominant kelp along Coastal areas of the North Atlantic, and they also occur
as an understory species in the températe Pacific.

Kelp not only are the largest of all algae, but some are considered to be
among the world’s fastest-growing plants. Growth rates of 6-25 cm per day
are common, and Macrocystis pyrifera grows as much as 50-60 cm per day
off California. Kelp can be either annual or perennial species, some
regrowing new stipes and blades from the original holdfast either yearly, or
every few years. All kelp reproduce via production of spores.

The high growth rates of kelp transíate into productivities of from about 600
to more than 3000 g C rrT 2 yr -1 (compare with production valúes for rocky
intertidal areas in Section 8.2.2). Off Amchitka Island in the Aleutians,
annual kelp production is from 1300 to 2800 g C m -2 . This high production
once supported populations of the giant Steller’s sea cow (see Section 6.4).
Off Nova Scotia, in Atlantic Cañada, Laminaria forests produce about
1750 g C m -2 yr -1 . Kelp beds off South Africa have a production of about
600 g C m -2 annually. In some locations, kelp are harvested for fertilizer,
iodine salts, industrial Chemicals, and alginates used as food additives. The
commercial harvest of the California giant kelp amounts to 10000-20000
tonnes dry weight per year.

Kelp communities provide spatial heterogeneity and diverse habitats and thus
support a highly diverse association of animáis. The large surface area of the
kelp blades provides space for numerous epifiora and epifauna, including
diatoms and other microflora, and colonial bryozoans and hydroids. A
variety of molluscs, crustaceans, worms, and other animáis live on the
plants, or on the substrate between plants. In some areas, much of the
primary production may be consumed by herbivores such as sea urchins.
Some snails and sea slugs (e.g. Aplysia) also feed on kelp directly, but
usually consume only vejy small amounts of the total production. This
habitat also supports a variety of fish that feed on kelp-associated animáis
and that find protection from predators such as seáis, sea lions, and sharks.

In many areas, however, as much as 90% of the kelp production is not
consumed but enters the detritus food chain. The edges of the kelp are
continually abraded by wave action, with small fragments being torn off, and

204

there may be self-thinning with some plants losing the competition for
available light, space, and nutrients. Annual kelp species (e.g.

Nereocystis spp.) may attain a summertime biomass of more than 100 tonnes
per hectare, all of which may be destroyed in the first winter storm. This
material enters the detrital pool of the kelp bed or is exported to other areas.
Kelp that is uprooted in storms may wash up in large quantities on beaches,
where it is eaten by amphipods or isopods. Kelp also release considerable
quantities of organic matter into solution, and this exúdate is utilized by
bacteria and thereby converted into particulate biomass (see Section 5.2.1).

The sea otter ( Enhydra lutris) (Colour Píate 32) is considered to be a
keystone species in North Pacific kelp forests. Otters eat a wide variety of
prey including crabs, sea urchins, abalone and other molluscs, and
slow-moving fish, and a single otter may eat 9 kg of food per day. Off
Amchitka Island, otters at a density of 20-30 km -2 annually consume about
35 000 kg krrr 2 of prey. Otter predation on sea urchins regulates the
ecological balance between kelp production and the destruction of the kelp
by the herbivorous urchins.

Sea urchins ( Stronglyocentrotus spp.) graze directly on living kelps, and they
are capable of eating through the holdfasts that anchor the algae to the
seafloor. The detached kelp is then swept away from the area in ocean
currents. Otters, by direct predation, maintain relatively low densities of sea
urchins and thus protect the kelp from overgrazing. The importance of otters
in maintaining healthy kelp forests was revealed from ecological
comparisons of different islands in the Aleutian chain. Certain islands off the
Alaskan coast were found to have lush kelp communities with thriving
populations of otters, seáis, and bald eagles; other nearby islands had no kelp
or otters, and few seáis or bald eagles. The depaupérate islands were the
focus of extensive hunting expeditions during the eighteenth and nineteenth
centuries, and historical records document the elimination of otters from
many kelp beds during this time. Only those islands which had been
repopulated with the few surviving otters had flourishing kelp forests. Since
1911, sea otters have been protected by law, and populations have recovered
in some areas of Alaska and British Columbia. They have also been
deliberately reintroduced into other depopulated regions, including along the
California coast. Where otter populations have recovered, sea urchin numbers
have decreased, and kelp production has increased. Despite protection, otters
remain a vulnerable species. They are disliked by fishermen, who often
perceive them as competitors for fish or shellfish (especially the valuable
abalone). Otters are also particularly vulnerable to oil spills, as the Exxon
Valdez experience of 1989 demonstrated when at least 5000 otters were
killed by exposure to crude oil spilled off the Alaskan coast.

Wherever sea urchins occur in very high densities in kelp beds, they are
capable of eliminating the kelp as illustrated in Figure 8.4. Prior to 1968
there were luxurious beds of Laminaria along rocky shores of the Nova
Scotian coast (eastern Cañada). The kelp beds extended to a depth of about
20 m, and they supported sea urchin populations of about 37 individuáis
m -2 . From 1968 onward, the urchins (Stronglyocentrotus droebachiensis)
became more and more abundant, and barren areas developed where the
kelps were eliminated. By 1980, urchin-dominated barren grounds extended
along more than 400 km of coastline. The rocky substrate became encrusted
with coralline red algae, which are not controlled by urchin grazing. In the
early 1980s, the urchin population was decimated by epidemic disease and

205

Figure 8.4 The biological events that altérnate
between productive kelp beds and barren
grounds that are dominated by sea urchins.

kelp began to reappear along most of the coast. Within three years of the
mass mortalities of urchins, luxuriant kelp beds had been re-established in
some areas.

Anecdotal evidence from fishermen suggests that mass mortalities of sea
urchins, and reciprocal fluctuations in kelp and urchin abundance, have
occurred off the Nova Scotian coast since at least the turn of the century.
The population explosions of sea urchins may be related to very high
recruitment success of larvae in certain years, and this may be related to
changes in local seawater temperature. As well, higher than average water
temperatures have been linked with outbreaks of disease in urchins. These
facts suggest that fluctuations in kelp and urchins may be natural events
triggered by environmental change, and that they may have been occurring
over a very long time. In any case, it is clear that urchin grazing and disease
regúlate the ecological dynamics of these subtidal Laminaria communities.

8.4 SANO BEACHES

The intertidal zones of sandy beaches appear barren when compared with
rocky shores. In particular, beaches exposed to severe wave action often
seem entirely devoid of life. This is because the nature of the substrate sets
living conditions which are best met by infaunal organisms that usually
remain hidden from direct observation. This makes sand beaches more
difficult to study than areas where the activities of the resident organisms can
be directly observed. Compounding the problem is the fact that many of the
organisms in this environment are of very small size, making their separation
from the sediment tedious, and their taxonomic identification difficult.

8.4.1 ENVIRONMENTAL CHARACTERISTICS

Beach sand grains are usually formed of irregularly-shaped quartz particles
mixed with a high proportion of shell fragments, and with detritus derived
from both marine and terrestrial sources. The particle size of sand varies
from <0.1 mm to >2 mm and is determined largely by the degree of wave
action; protected beaches have finer sand particles than exposed areas, where
waves resuspend and transport small-sized grains. There is a gradient in
particle size between sand and mud, with mud being composed of finer

206

partióles and mudflats being formed in areas of little water movement (see
Section 8.5). Some substrates are difficult to characterize, giving rise to
terms like muddy sand, or sandy mud. As partióle size increases, sand grades
into gravel or shingle. These large-particle substrates do not retain water
because of their high porosity, and the shifting and abrasión of the large
partióles also contribute to an absence or low diversity of life on gravel and
shingle shores.

Sandy beaches typically have a gradual slope, and this means that the
sediment drains and dries out relatively slowly. Although oxygen is plentiful
in the overlying water, oxygen contení in the substratum diminishes with
depth because of the respiration of micro-organisms and the oxidation of
Chemicals within the sand. Anaerobio conditions are marked by a black
sulphide layer beginning at a depth of from a few millimetres to nearly a
metre, depending on the organic content of the sand. Chemosynthetic
bacteria are present in the sulphide layer (refer to Section 5.5).

Animáis require special adaptations to live in an environment where the
substrate is physically unstable in the sense that the sand grains are
continually moved by turbulent water. The continual shifting of the surface
layers of exposed beaches exeludes large sessile species and most large
epifauna in general. As well, sand beaches contain relatively low
concentrations of organic matter. On the other hand, sand buffers against
large temperature and salinity fluctuations, and organisms burrowing into
sand are kept moist at low tide. Sand also acts as a protective cover from
intense solar radiation. Although there are differences in the physical
environment and in the distribution of species from high to low tidal marks,
the zonation patterns are not as clearly obvious as on rocky shores. Zonation
on sandy beaches is also dynamic and variable; as the tide rises, many
populations change their positions on the shore, or enter the water column.

8.4.2 SPECIES COMPOSITION

Primary producers

There are no large attached plants below the high tide mark on sand beaches.
The dominant benthic primary producers are diatoms, dinoflagellates, and
blue-green algae. These are restricted to the near-surface layers of the
sediment because light does not penétrate very deep in sand. The primary
productivity of these benthic plants is very low (<15 g C m~ 2 yr -1 ), and the
system depends primarily on energy derived from primary productivity in
the surrounding water and on organic detritus.

Macrofauna

There is a low diversity of macrofauna compared with rocky shores or mud
communities. Burrowing polychaetes and bi val ves plus crustaceans are
usually the dominant members in terms of biomass. In températe latitudes,
the supralittoral zone is occupied by air-breathing amphipods (beach hoppers
or beach fleas) and, sometimes, also by isopods. Amphipods and isopods
burrow into the sand during the day, and feed at night on detritus like
decaying seaweed that has washed ashore. On tropical sand beaches, the
highest reaches are occupied by ghost crabs ( Ocypode ) which are also
scavengers.

The mid- and lower tidal zones support a higher diversity of macrofauna.
Small, fast-burrowing wedge-shaped clams ( Donax , Tellina) are often present
in vast numbers, some migrating up and down the beach with tidal changes.

207

Larger razor clams (e.g. Ensis , Siliqua ) are also confined to sandy shores,
and they too are rapid diggers. These mobile bivalves tend to have smooth,
thin shells with slender profiles that ease passage through sand. Stouter
thicker-shelled bivalves like cockles (e.g. Cardium) or Macoma also inhabit
sand, but they tend to anchor themselves more firmly in the sediment and
move less. The bivalves may be either suspensión feeders or deposit feeders,
with some species capable of taking advantage of both food sources. In
general, deposit feeders tend to dominate in fine-particle sands, presumably
because the concentration of organic material is higher than in coarser sands.
The sand environment is also home to certain snails that plough through the
sand; these inelude the olive shells (e.g. Olivella ), and the larger moon snails
(. Natica , Polinices ), all with smooth, undecorated shells. The majority of
olive shells prey on small molluscs. Moon snails are also predators,
especially of bivalves, and they gain access to their prey by drilling a hole
through the shell. Where they are abundant, moon snails can have important
effeets on community structure; when Polinices is experimentally removed,
the numbers of clams and other infaunal prey increase.

Few animáis form permanent burrows because wave action and the relatively
large particle size of sand cause their collapse. However, exceptions can be
found in those polychaete species that line their burrows with mucus or
membranous materials. Many of the sand-dwelling polychaetes are deposit
feeders, a few are suspensión feeders on plankton or resuspended organic
material, and some (e.g. Nephthys, Glycera) are predators or scavengers that
move through the sand actively seeking food.

Characteristic crustacean inhabitants of mid-tidal levels inelude mole crabs
of the genus Emérita . They typically lie with their entire body buried and
only their antennae projecting above the surface of the sand to capture small
suspended food particles from receding waves. Despite being swift
burrowers, they are commonly preyed upon by shore birds. Prawns and
mysids are other sandy-shore crustaceans; they burrow temporarily but
emerge to feed. Predators of this community can inelude larger
epifaunal crabs.

Various types of echinoderms may be present at lower tidal levels, including
burrowing sea cucumbers, heart urchins, and sand dollars, most of which are
deposit feeders. The shortened spines of the sand dollars facilitate
burrowing, and the young of some species ingest and selectively accumulate
the heaviest sand particles containing iron oxide in their digestive tract.

These increase the density of the small sand dollars and act as a weight belt,
keeping them in the sediments when wave action is intense. Starfish are not
common inhabitants in températe communities, but some tropical species
(e.g. Oreaster) are present in low intertidal or subtidal depths where they
feed on organic matter contained in the sand.

Vertébrate members of sand beach communities inelude fish that are
permanent residents, like sand eels that burrow into the sand at low tide, and
temporary inhabitants (e.g. flatfish) that move into the area only at high tide
to feed on smaller animáis. Shorebirds are also important predators of the
community, and mammals (e.g. rats, otters) may visit this area to obtain food.

Meiofauna

The meiofauna of sand beaches ineludes some of the most diverse and
highly adapted species in this environment. The term interstitial fauna is
also applied to those animáis that live in the interstices, or spaces, between

208

the sand grains. They either attach to sediment partióles, or move through
the interstitial spaces without dislodging the grains. Many animal phyla are
represented in this category, and some groups (e.g. gastrotrichs, see
Figure 8.5c,d) are wholly or largely restricted to this particular environment.
Biomass of the meiofauna usually ranges between 1 and 2 g m~ 2 , with the
average number of individuáis being 10 6 m -2 (from the sand surface down
to the anoxic layer).

Figure 8.5 illustrates some of the characteristic meiofauna of sand. Many of
their adaptations are morphological and can be seen in the figure; these
inelude small size (only a few mm in largest dimensión even among groups
that are usually large, such as echinoderms and molluscs), elongate or
wormlike shapes, and flattened bodies. As well, many have a strengthened
body wall as protection against crushing in a physically unstable substrate.
This may involve the development of spines or scales (e.g. in gastrotrichs), a
well-developed cuticle or exoskeleton (as in nematodes or crustaceans), or an
internal skeleton of calcareous/spicules (some ciliates and sea slugs).
Alternatively, soft-bodied animáis like ciliates, flatworms, and hydroids have
developed the ability to contract strongly to protect against mechanical
damage. Many of the interstitial species have special adhesive organs for
maintaining a hold on the sediment particles; these may be epidermal glands,
hooks, or claws (note Figure 8.5e in particular).

The majority of the sand meiofauna are mobile, but some foraminiferans and
hydroids (Figure 8.5h) remain firmly attached to sand particles. All feeding
types are present, from animáis like ostracods and harpacticoid copepods that
graze on benthic diatoms and dinoflagellates, to detritus feeders (gastrotrichs,
nematodes), to predators such as hydroids and flatworms. Suspensión feeders
are the rarest type, and these are sedentary animáis like bryozoans and

Figure 8.5 Representative meiofauna from
sand, all between 0.1 and 1.5 mm in length.

(a) Psammodriíus (a polychaete);

(b) Monobryozoon (a bryozoan attached to sand
grains); (c) Dactylopedalia (a gastrotrich);

(d) Urodasys (a gastrotrich); (e) Batillipes (a
tardigrade); (f) Unela (a gastropod mollusc);

(g) Pseudovermis (a gastropod mollusc);

(h) Psammohydra (a hydroid attached to sand
particles); and (i) Nerillidium (a polychaete)

209

tunicates. The meiofauna fall prey to macrofaunal deposit feeders, shrimp,
and young fish.

Fecundity of the meiofauna is low owing to their small sizes and the
consequent physical constraints on producing large numbers of gametes.
Many of the species produce only one to ten eggs at a time, and about 98%
of the species lack pelagic larvae. The young are often brooded by the parent
until they are able to live freely or, alternatively, eggs are attached to the
sand and the young hatch as benthic juveniles. Dispersal is by passive
transport of those eggs or adults that are caught in water currents when the
sand is washed away, or by organisms attached to sand partióles that adhere
to the feet of wading birds.

QUESIIQN 8.2 Aboui 98% of the meiofauna in sand do not produce
planktonic larvae. What factors favour direct development and suppression
of a pelagic phase in these species and in this environment?

8.5 ESTUARIES

Estuaries are partially enclosed regions where large rivers enter the sea. They
rank among the most productive of marine ecosystems as they typically
contain a high biomass of benthic algae, seagrasses, and phytoplankton, and
support large numbers of fish and birds. Estuaries are enriched by nutrients
from land drainage, but their high productivity is also the result of nutrient
retention within the estuary. This is due to the water circulation pattern that
is set up when less dense freshwater overlies heavier salt water. Figure 3.15
illustrates how estuaries tend to entrain nutrients from deep, saline water into
the freshwater flowing seaward from the river, with the nutrient enrichment
usually leading to a phytoplankton bloom seaward of the river mouth. Some
of the bloom will sink out into the lower, more saline layers, and the
decomposing phytodetritus will then be carried back toward the land. Thus
the special circulation pattern of estuaries, combined with tidal flow, results
in the sinking of particles and nutrients from seaward-flowing river water,
and in these nutrients being carried back at depth in the saline water that
fiows inward and upwells to replace that carried away by the surface flow.

Each estuary has unique physical features that influence its ecology. These
inelude the amount of river discharge, depth and general topography, specific
circulation patterns, climatic regime, and vertical tidal range. Nevertheless,
certain generalities emerge from the many comparative studies of life in
estuaries. In several respeets, the estuarine ecosystem is much more complex
than open ocean ecosystems, and the plankton community at the seaward
edge of the estuary is only one of several communities governed by different
groups of primary producers. The major components that typically make up
estuaries are illustrated in Figures 8.6 and Colour Píate 33; the relative area
occupied by each of these communities depends on local tidal action and the
topography of the estuary.

Starting from the upper reaches of temperate-latitude estuaries, there is firstly
a sheltered, upper intertidal saltmarsh community dominated by a variety of
marshgrasses (e.g. Spartina , Salicornia)\ this community is largely replaced
by mangroves in tropical and subtropical latitudes (see Section 8.7). The
marshgrasses, which are rooted flowering plants, may be as much as 2 m
high, and they function as a trap for nutrient-rich sediment. Above-ground

SALTMARSH

ESTUARINE

COMMUNITIES

SEAGRASS BED MUDFLAT/ PELAGIC
SANO BAR

litó"''""::;::':

- SUBTIDAL

%m ^

Primary

Marshgrasses

Seagrasses

Epipsammic

Phytoplankton

producers

or mangroves

and seaweeds

afgae

Secondary

Birds and

Snails and

Meiofauna

Zooplankton

producers

inseets

crustaceans

Higher

Birds, mammais,

Benthic

Fish

trophic

reptiles, etc.

macrofauna

leveis

and fish

Human

Airports,

Harbour development,

Disposal site for

impacts

housing,

including dredging,

etc.

sewage, dredged

agriculture, etc.

materials, etc.

Figure 8.6 A schematic depiction of the communities composing the estuarine ecosystem,
showing their dominant flora and fauna and potential human impacts.

primary production of marshgrasses ranges from 200 to 3000 g C rrT 2 yr -1 ,
and production by benthic mud algae contributes another 100 to
600 g C m -2 yr -1 . Thus saltmarshes rank among the most productive
ecosystems on Earth. Most of the living plant material is not grazed directly,
but enters detritus food webs either on the marsh or in adjacent waters. This
plant debris decays slowly and, over long periods, the accumulation of debris
and trapped sediment may create peat deposits that are several metres deep.
This upward growth of saltmarshes results in changes in relative tidal level
and drainage, and thus in changes in the species composition of plants; this
process of marsh evolution eventually contributes to the infilling of estuaries.

The upper reaches of a saltmarsh mark the transition between the sea and
land. This habitat has great variations in salinity and temperature, and
relatively few species of plants and animáis live here permanently.

Terrestrial animáis such as raccoons, rats, and snakes invade this area, and
there are large insect and bird populations. Faunal diversity is greater in the
lower intertidal areas of the marsh, and the saltmarsh macrobenthos may
inelude deposit-feeding fiddler crabs ( Uca) that build burrows in the mud,
snails (e.g. Nassarius, Hydrobia, Littorina) that feed on the rich deposits of
benthic diatoms, and mussels of the genus Modiolus that are specially
adapted to live in or on mud and that can respire in both air and water. The
leaves and stems of the marshgrasses serve as attachment sites for many
small organisms, and significant numbers of micro- and meiobenthos live on
or in the bottom sediments. Bacteria attain densities as high as 10 9 cm" 3 in
the sediments, and they are an important food for protozoans and meiofauna.
Saltmarshes fulfil the important function of providing shelter and food for
shrimp, juvenile lobsters, and the young stages of many species of marine
and estuarine fish.

A seagrass community is located seaward of the saltmarsh, in the intertidal
and subtidal zones. It may contain significant stands of seaweeds in addition
to the seagrasses, but in general, seaweeds do not grow as well in muddy
estuarine waters as they do in clear waters. The dominant plant of this
estuarine community in températe latitudes is Zostera , commonly called
eelgrass; in tropical climates, it is replaced by Thalassia , or turtlegrass, The
brown seaweed Fucus and green seaweeds, Enteromorpha and Ulva , may
grow on patches of rock among the seagrass beds. Measurements of the

211

productivity of seagrasses are complicated by the fact that many epiphytic
diatoms may grow on the blades of the seagrass, and these may add to the
total primary productivity. For example, on the eastern coast of the United
States, Zostera may produce about 350 g C m~ 2 yr" 1 , and associated plants
contribute a further 300 g C m -2 yr -1 . Generally, the annual production of
températe seagrasses is about 120-600 g C m -2 , while tropical seagrass
communities have higher net primary productivities of up to about
1000 g C m -2 yr -1 .

Numerous meiofauna, including protozoans and nematodes, are associated
with the seagrass epiphytes which are grazed by snails, isopods, amphipods,
and harpacticoid copepods. Sessile filter-feeding invertebrates (e.g. hydroids,
bryozoans, and tunicates) attach to the seagrass leaves. Snails, bivalves,
polychaetes, and various types of crustaceans dominate the mobile
invertebrate fauna of seagrass communities. This estuarine zone, like the
saltmarsh, serves as a nursery area for the young of many species of fish,
including commercial species such as menhaden and salmón.

In both the saltmarsh and seagrass communities, little of the primary
production is consumed by herbivores. Both communities are dominated by
detritus-based food chains because marshgrasses and seagrasses contain large
amounts of refractory material, such as cellulose, that is difficult for
herbivores to digest. Less than 10% of the marshgrass is grazed by terrestrial
herbivores, and usually only a small fraction of the seagrass production is
eaten by such animáis as sea urchins and migrant birds (e.g. geese).
However, in some tropical regions, turtlegrass may be consumed in large
quantities by dugongs or manatees, and by sea turtles. In general, though, by
far the largest fraction of the net primary production in both communities
dies and is colonized by fungi and bacteria, to be converted eventually into
microbial biomass. The numbers of bacteria in estuarine water are much
higher than in seawater, and bacterial densities in sediments may reach
200-500 x 10 6 per gramme of estuarine mud. Thus a large amount of plant
detritus is produced, some of which is exported out of the estuary, and much
organic carbón is recycled to re-enter the food chain through the microbial
loop (see Section 5.2.1). Within the sediments, much of the organic matter is
decomposed under anoxic conditions, with anaerobic bacteria using primarily
inorganic sulphate as a source of oxygen.

QUEST1QN 8.3 Is the occurrence of hydrogen sulphide in sediments an
indicatíon of pollution?

On the seaward side of the seagrasses, either a subtidal mudflat or sand-bar
community will be present depending on the current and tidal regime. In
fact, this community is continuous underneath both the intertidal seagrass
and saltmarsh communities. The dominant primary producers of mudflats or
sand bars are the epipsammic algae, which are generally species of benthic
diatoms or dinoflagellates that are specially adapted to grow on sediment
partióles. The surface of mud is sometimes colonized by thick mats of
filamentous blue-green algae of several types. The productivity of this región
tends to be inversely correlated with the grain size of the sediment particles,
so that mudflats are generally more productive than sand bars in the same
location. The primary productivity of these communities (in the absence of a
cover of marshgrasses or seagrasses) tends to be low. Sand bars have a
primary productivity of about 10 g C m -2 yr" 1 but mudflat production by
benthic microphytes may be as high as 230 g C m -2 yr -1 .

number of species

212

QUESTI0N 8.4 Why should ihe partióle size of sand vs. mud affect the
prodüctivity of the epipsammic algae?

0 5 10 15 20 25 30 35

saliníty

Figure 87 An idealized diagram of the
distribution of freshwater, brackish-water, and
marine animáis relative to salinity. (Numbers of
species given in relative units.)

The mudflat community supports a wide range of animáis, with crabs and
flatfish being common epifauna, and bivalves, polychaetes, and mud shrimp
dominating the infauna. There is a rich meiofauna of small copepods,
nematodes, and polychaetes, and an equally rich microfauna of protozoans,
especially ciliates. Detritivores are usually predominant in the community. In
the shallower reaches of this community, large numbers of birds eat the
detritivorous invertebrates. Food consumption by birds may represent a
significant impact on the mudflat species. For example, each of several
thousands of knots ( Calidris canutus) on a large mudflat may eat as many as
730 small clams ( Macoma) per day; a single redshank ( Tringa totanus ) may
consume up to 40000 burrowing amphipods ( Corophium); and one
oyster-catcher ( Haematopus ostralegus) can eat 315 cockles ( Cardium )
daily. Overall, birds may take between 4% and 10% of the accessible
invertebrate fauna.

The pelagic community located on the seaward edge of the estuary is
controlled largely by the primary prodüctivity of the phytoplankton, and this
ranges from about 100 to 500 g C m~ 2 yr -1 , depending on water clarity.
Although nutrients may be plentiful, turbidity of the water often restricts
light penetration and limits phytoplankton production. In shallow estuaries,
as much as half of the phytoplankton may be consumed by filter-feeding
benthos, with the rest being eaten by zooplankton. Zooplankton also may
feed on benthic diatoms and bacteria-covered sediment partióles that are
resuspended by intense mixing in shallow estuaries. In deep fjordlike
estuaries, benthic plants are light-limited and most of the estuarine primary
production is carried out by phytoplankton.

Although estuaries are highly productive and host many juvenile fish, as well
as large numbers of crustaceans, molluscs, shorebirds and waterbirds, the
number of species found in these areas is relatively small compared with
other marine habitats. Few species are adapted to cope with the salinity,
temperature, and turbidity variations present in this habitat. Salinity tolerance
plays a major role in the distribution of any particular species in an estuary,
although distribution is also determined by such factors as substrate type and
degree of tidal exposure.

Figure 8.7 illustrates the typical distribution and relative diversity of
freshwater, brackish, and marine animáis in relation to estuaries. Estuaries
support an essentially marine fauna, but the number of marine species
declines as the water becomes less saline, and the species change from those
that are stenohaline to those that are euryhaline (see Section 2.3.2). The
majority of animáis living in rivers do not tolérate salinities greater than
about 0.5, and they do not penétrate further seaward than the uppermost
reaches of the estuary. Only a few freshwater organisms (oligohaline
species) can survive in water having a salinity of 0.5 to about 5; these
inelude principally various insect larvae, oligochaete worms, snails, and
some fish such as sticklebacks. There are relatively few brackish-water
species that are restricted to estuarine conditions with salinities of about
5-20, and most are animáis with marine affinities. Euryhaline marine
organisms constitute the majority of species living in estuaries, and their
distributions extend from the sea into the central regions of estuaries.

213

Stenohaline marine species are unable to tolérate salinities lower than about
25-30, and they are largely excluded from estuaries. Some fish (e.g. salmón,
eels) are transient residents of estuaries, and move freely from the sea to
rivers and lakes, or vice versa (see Section 6.6.1). Overall, estuaries have
fewer species than adjacent aquatic environments, but abundance within
individual species as well as biomass are often markedly increased.

In general, the extent of penetration into estuaries by marine and, conversely,
freshwater species is determined by the rate and magnitude of tidal change,
rather than by the salinity gradient. That is, marine species penétrate farther
upstream, and freshwater organisms reach much closer to the sea, in
estuaries where tides are small and the salinity gradient is relatively stable.
The minimum number of species occurs in that part of the estuary where the
salinity variation is greatest. Finally, the distributions of benthic species
within estuaries are also controlled by sediment type.

QUESTION 8,5 Can you offer any explanation(s) as to why species diversity
declines in estuaries relative to adjacent environments, but numbers of
individuáis and biomass increase?

8.6 CORAL REEFS

Coral reefs are well known for their spectacular beauty (Colour Plates 34
and 35), and they are perhaps the most diverse and ecologically complex of
marine benthic communities. They are unique in being formed entirely by
the biological activity of certain coráis belonging to the Phylum Cnidaria
(see Table 7.1). These tropical reefs result from massive deposits of calcium
carbonate laid down by the coráis o ver aeons of geologic time. These are
among the oldest of marine communities, with a geological history
stretching back for more than 500 million years.

8.6.1 DISTRIBUTION AND LIMITING FACTORS

Living coral reefs cover about 600 thousand km 2 , or somewhat less than
0.2% of the global ocean area and about 15% of the shallow sea areas within
0-30 m depth. The largest reef is the Great Barrier Reef that extends along
the east coast of Australia for a distance of more than 2000 km and is as
much as 145 km wide. Reefs are located exclusively within water bounded
by the 20°C isotherms and so are virtually confined to the tropics
(Figure 3.10). Reef-building coráis cannot tolérate water temperatures of less
than 18°C, and optimal growth usually occurs between 23° and 29°C,
although some coráis tolérate temperatures of up to 40°C. A number of other
physiological demands further limit the distribution of reef-building coráis.
They require high salinity water ranging from 32 up to 42. High light levels
are also necessary for reef-building (for reasons that will be explained
below), and this restricts coráis to the euphotic zone. Even in the clear
oligotrophic water of the tropics, most reef-building species live in water
that is shallower than 25 m. The upward growth of a reef is restricted to the
level of lowest tides, as exposure to air for more than several hours kills
coráis. Coráis are also absent in turbid waters, as they are very sensitive to
high levels of suspended and settling sediment which can smother them and
clog their feeding mechanisms. High turbidity also affects reef-building by
decreasing the depth of light penetration. New reefs are initially formed by

the attachment of meroplanktonic coral larvae to a hard substrate, and for
this reason reefs always develop in association with the edges of continents
or islands.

QUESTION 8.6 Refer to Figure 3JO. (a) Can you explain why coral reefs are
generally absent on the west coasts of the Americas and Africa betvveen 30 S
and 30 N? íb> Whai might preverá reef formación off nonh-easiern South
America, northward froni the mouths of the Amazon and Orinoco rivers?

8.6.2 CORAL STRUCTURE

Coráis are closely related to benthic sea anemones (both are in the Class
Anthozoa) and are more distantly related to planktonic jellyfish, benthic
marine hydroids, and the freshwater Hydra . Not all coráis are reef-builders;
some are solitary or colonial animáis that are capable of living in deeper
and/or colder water and are found throughout the world’s oceans.
Reef-building stony coráis are colonial animáis, and each reef is formed of
billions of tiny individuáis called polyps (Figure 8.8; Colour Píate 36). Each
polyp secretes a calcium carbonate exoskeleton around itself that generally
measures about 1-3 mm in diameter. Each polyp is equipped with tentacles
containing batteries of nematocysts (see Section 4.2), and these stinging cells
can be used to capture prey and for defence. The polyps can produce a large
colony by asexual división, or budding, and all the polyps in a colony
remain connected to each other by extensions of their tissues. Coráis also
reproduce sexually, producing planktonic larvae that disperse, settle, and
establish new colonies.

Individual coral colonies vary in size, but some are very large, weighing up
to several hundred tonnes. The form of a colony, whether it is branching,
massive, lobed, or folded, depends on the species and also on the physical

tentares

Figure 8.8 Anatomy of a coral polyp. The animal is basically a contractile sac housed in a
carbonate skeleton. The central mouth is surrounded by six, or a múltiple of six, tentacles
equipped with batteries of nematocysts. The tiny zooxanthellae live in cells in the lining of
the central digestive cavity. Each polyp secretes a protective carbonate exoskeleton consisting
of a radial arrangement of vertical plates; as it grows upward, the polyp deposits new layers
under itself.

215

environment in which the coral is located. The same species may have a very
different form when it grows in areas exposed to wave action as opposed to
calm conditions, or when it grows in shallow versus deeper waters.

8.6.3 DIVERSITY

The diversity of Ufe on a coral reef is extraordinarily rich. Figure 8.9
illustrates only a very few dominant types of the coral-reef fauna. The Great
Barrier Reef is composed of about 350 species of hard coráis, and is home
to more than 4000 species of molluscs, 1500 species of fish, and 240 species
of seabirds. In addition, there are many more species of macrobenthos, and
the numbers of micro- and meiofauna remain unknown. Representative
species of almost all phyla and classes can be found in the reef ecosystem.

Figure 8.9 A coral reef habitat illustrating some of the many inhabitants of this diverse
ecosystem.

petrel

snail

jellyfish

nudibranch (sea slug)

angelfish

sponges

lobed coráis

colonial tunicate

sea whips (gorgonian coráis)

giant clam ( Tridacna)

triggerfish

pseudochromid fish

sea fans (gorgonian coráis)

starfish

tube anemone

soft coráis

stone coral

cleaner shrimp

bryozoans

sea anemones

brain coral

clownfish

butterfly fish

worm tubes

moray eel

snail (cowry)

cleaner fish

sea fan (gorgonian)

tube coráis

216

Reefs in the Indo-Pacific have a high diversity of coral species, with at least
500 reef-building species throughout the entire región. Atlantic reefs are
impoverished in comparison, with only about 75 species of reef-building
coráis. The number of species in other animal groups associated with reefs is
also generally lower in the Atlantic sector than in the Indo-Pacific. The
number of mollusc species is estimated at about 5000 in the Pacific versus
1200 in the Atlantic, and there are about 2000 versus 600 fish species in
these respective reef areas. The differences in species diversity may result
from differences in the age of the oceans, and the respective geologic times
over which reefs have evolved. Geologically, the Atlantic is a more recent
ocean, and its reefs were also more severely influenced by decreased
temperatures and lowering sea levels during ice ages. Most Atlantic reefs are
only 10000-15 000 years oíd, these dates corresponding to the last glacial
age. In contrast, the Great Barrier Reef is about 2 million years oíd, and
some Pacific atolls date back about 60 million years.

The reef itself provides food and shelter for many plants, invertebrates, and
fish. For sessile species, the reef offers a site of attachment. Surface
irregularities in the reef limestone create a variety of microhabitats like
crevices and tunnels, and these contribute to the faunal diversity of the
system. Areas of rubble and sand also accumulate between coral heads, and
these sediment types require different sets of adaptations and develop
different communities from that associated with the hard-substrate reef. A
reef is also differentiated into regions distinguished by physical differences
in wave action, depth, and degree of tidal exposure. This wealth of different
habitats is a major factor in supporting the many species of a reef.

Coral polyps usually dominate the living biomass of a reef, but other reef
organisms also contribute to the carbonate reef structure. These inelude the
hard, coralline red algae that grow in thin layers over the surface of the reef.
These encrusting algae precipítate CaCC >3 and play a role in cementing the
reef fragments together. Some green algae also secrete calcium carbonate,
other green algae do not. In addition to encrusting algae, there are benthic
algae that are erect species, and some that live within the spaces of the coral
framework. Seagrasses often grow in the sandy areas within or surrounding
the reef. All of these plants provide food for herbivorous species of
invertebrates and fish. However, the algae are generally inconspicuous
inhabitants of the reef, and animal life is visually dominant.

In addition to the reef-building stony coráis, other types of cnidarians are
prominent reef members (see Figure 8.9 and Colour Píate 37). These inelude
several types of non-reef-building coráis, including tire coráis, pipe coráis,
and soft coráis. Sea whips and sea fans are also common reef inhabitants;
they are cióse relatives of stony coráis and have internal skeletons composed
of calcareous spicules. Other major in vertébrate groups in a reef community
inelude echinoderms (starfish, sea urchins, and sea cucumbers), molluscs
(limpets, snails, and clams), polychaete worms, sponges, and crustaceans
(including spiny lobsters and small shrimp). Some of the invertebrates are
encrusting species, like bryozoans; some build calcareous tubes, like certain
polychaete worms; and some snails attach tube-like shells to the reef. All of
these activities serve to cement the limestone reef framework together. In the
Pacific, giant clams belonging to the genus Tridacna are also important
structural components of reefs (Figure 8.9). These molluscs contribute an
astonishing biomass to the reefs, as they grow to over 1 m in length and
may exceed 300 kg in weight.

217

Fish comprise the dominant vertebrates on a reef. Many of the reef fish are
brightly coloured and visually conspicuous. About 25% of the world’s
species of marine fish are found only in reef areas. These diverse species of
fish show a high degree of feeding specialization and food selection. Some
are herbivores, feeding on algae or seagrasses; some specialize in being
plankton-feeders; and some are piscivorous, or are predators of benthic reef
in vertebrates. Fish not only play important ecological roles in grazing or
predation, but the faeces of these abundant animáis contribute an important
source of nutrients to the reef ecosystem.

The very large number of reef species, and the abundance of life on the reef,
lead to intense competition between species and between individuáis for
limited resources. The high degree of food specialization observed in many
reef species is a reflection of the high species diversity of the reef, and every
available food resource is efficiently utilized. There is also intense
competition for space on the reef, and every microhabitat is occupied by
organisms adapted to their particular site. Experimental work has revealed
that the mesenterial filaments (Figure 8.8) of some coráis contain substances
that kill polyps of adjacent colonies. Aggressive, slow-growing coráis can
thus avoid being overgrown by less aggressive, but faster-growing species.

8.6.4 NUTRITION AND PRODUCTION IN REEFS

Reef-building coráis are also called hermatypic coráis. They are
distinguished from non-reef-building (ahermatypic) species by having a
special symbiotic association with certain algae. Each hermatypic coral
polyp contains masses of photosynthetic dinoflagellates, called
zooxanthellae. These are a vegetative form of free-living dinoflagellates;
when cultured under laboratory conditions, they develop into motile
flagellate forms identical with planktonic dinoflagellates (see Section 3.1.2).
The zooxanthellae in all coráis belong to a single genus, Symbiodinium , with
different species or strains being specific to particular coral species. The
zooxanthellae Uve within cells in the lining of the gut of coráis, reaching
concentrations of up to 30000 cells per mm 3 of coral tissue. Under stressful
environmental conditions, the symbiotic algae can be expelled from the
coral. Because much of the colour of coráis is due to the pigmentation of the
zooxanthellae, this expulsión is referred to as ‘bleaching’.

The algal-coral relationship is beneficial to both species. The coral provides
the algae with a protected environment, but it also provides certain Chemical
compounds that are necessary for photosynthesis. Carbón dioxide is
produced by coral respiration, and inorganic nutrients (ammonia, nitrates,
and phosphates) are present in waste producís of the coral. In return, the
algae produce oxygen and remo ve wastes; but most importantly, they supply
the coral with organic producís of photosynthesis that are transferred from
the algae to the host. These Chemical producís inelude glucose, glycerol, and
amino acids, all compounds that are utilized by the coral polyps for
metabolism or as building blocks in the manufacture of proteins, fats, and
carbohydrates. The symbiotic algae also enhance the ability of the coral to
synthesize CaCCT}. Rates of calcification are significantly slowed when
zooxanthellae are experimentally removed from coráis, or when coráis are
kept in shade or darkness. The relationship between the two independent
processes of CO 2 fixation by photosynthesis and CO 2 fixation as CaCC >3 is
complex and not fully understood. However, the symbiotic association with
photosynthetic dinoflagellates explains why hermatypic coráis require clear.

218

lighted water. This association also leads to intense competition for space
within areas of sufñcient light to support the zooxanthellae.

qUESTION 8.7 Can any coráis grow below the euphotic zone?

The coral-zooxanthellae symbiosis is maintained over time and distance
because the algae are already contained in coral larvae before they are
released from the parent polyp. This relationship is not unique on the reef,
however. Zooxanthellae are also present in other reef inhabitants, including
the majority of other cnidarians, some tunicates, some shell-less snails, and
in the giant clam Tridacna.

The symbiotic arrangement between algae and coráis or other invertebrates
results in nutrients being tightly recycled within coral reefs. This intemal
nutrient cycling is of primary importance in maintaining the productivity of
the reef in oligotrophic tropical water.

Symbiotic algae do not supply all the nutritional requirements of their hosts.
All the animáis harbouring zooxanthellae are mixotrophic and capable of
meeting their additional nutritional needs in other ways. Coráis are true
carnivores that capture zooplankton, employing their nematocysts to paralyse
the prey. Many coral species can also feed on suspended particles by
producing mucous nets or mucous filaments to entangle food that is then
drawn to the mouth by rows of cilia. Ciliary-mucus feeding extends the size
range of potential food Ítems to inelude even bacteria. Coráis may also
directly absorb dissolved organic matter.

The relative importance of zooxanthellae versus captured particulate food to
the nutrition of any particular coral probably depends on the particular
species, and it will be influenced by the specific Chemical that is produced
and translocated from the symbiotic algae to the host. It should also be
influenced by various environmental parameters such as depth, light
intensity, abundance of zooplankton, etc.

8.6.5 PRODUCTION ESTIMATES

Primary production in the coral reef System is carried out by the benthic
algae attached to or associated with the reef, by the suspended
phytoplankton, and by the zooxanthellae living within the animáis of the
reef. This ecological fractionation of primary producers makes accurate
measurements of primary productivity extremely difficult because different
techniques must be employed for each. With the exception of the
phytoplankton, it is also difficult to assess the standing stock of primary
producers. To do so requires determining the plant/animal proportions of
coral polyps and the relative contributions of various types of benthic algae
to total reef biomass. Until these have been determined, the size of the
primary producer trophic level remains uncertain.

Production studies of coral reefs suggest that gross primary productivity
ranges from about 1500 to 5000 gCm" 2 yr -i , valúes that are much higher
than those of open tropical oceans (see Section 3.5 and 3.6). In fact, they
represent some of the highest rates of primary production of any natural
ecosystem. However, many of the nutrients contributing to this production
are recycled (i.e. the /-ratio <0.1, see Section 5.5.1). Symbiosis between
primary producers and dominant animal species of the community, with

219

nutrients prevented from being washed away, is a dominant controlling
feature of the biological production, just as it is in the deep-water,
sulphide-communities which will be described in Section 8.9.

Net primary production on reefs is lower than might be expected because
respiration of the primary producers is high, with gross production to
respiration ratios ( P/R ) usually ranging from 1.0 to 2.5. In comparison,
healthy phytoplankton ha ve a P/R ratio of about 10. In addition, the
coral-reef food chain is much longer than in upwelling zones (see
equation 5.2), so that respiration losses throughout the entire ecosystem are
high. This results in lowering the production of top-level predators relative
to the high gross primary productivity.

8.6.6 F0RMATI0N AND GROWTH OF REEFS

During the voyage of the Beagle in the 1830s, Charles Darwin observed that
there were three basic types of coral reefs, and he formulated an hypothesis
of reef formation that linked these types. His ideas are summarized below
and illustrated in Figure 8.10.

Reef formation is initiated with the attachment of free-swimming coral
larvae to the submerged edges of islands or continents. As the coral grows
and expands, a fringing reef is formed as a band along the coast or around
an island. This type of reef is predominant in the West Indies (Caribbean
Sea). It is also the first stage in the process of forming atolls.

If the fringing reef is attached to the edges of a volcanic island or other land
mass that is slowly sinking, while the coral continúes to grow upward, a
barrier reef will eventually form. Rarrier reefs are separated from the land
mass by a lagoon of open deep water. The Great Barrier Reef of Australia is
the best known of this type, but it is in fact an aggregation of many reefs.

Atolls mark the last stage in this geological process. When a volcanic island
subsides below sea level, the coral reef is left as a ring around a central
lagoon. Continued coral growth maintains the circular reef, but calm
conditions and henee increased sedimentation in the central lagoon prevent
development of a reef in this area. Hundreds of coral atolls are found
throughout the South Pacific Ocean, all of them located far from land but
attached to underwater seamounts (volcanic elevations rising from the
seafloor) which have subsided with age.

Darwin’s ideas on atoll formation were not substantiated until the 1950s,
when drilling programmes on coral atolls encountered volcanic rock
hundreds of metres below the surface. His hypothesis has been further
supported by the discovery of seamounts, submerged far below the sea
surface, that still have attached remnants of shallow-water coráis.

QUESTIDN 8.8 Excluding pollution influentes, would you expect to tínd a
difFerence in tola i biological production between a barrier reef located
ofíshore of a continent and a mid-oceanic atoll? Explain your answer.

The rate at which a reef develops depends on a balance between the growth
rates (budding) and calcification of the coral polyps and the rates of
destruction of the limestone framework. Coráis always grow upward, toward
light, as each polyp deposits new carbonate layers under itself (Figure 8.8).
Growth of the coral skeleton is much faster in sunlight than in darkness (and

220

(a) fringing reef

sea

leve!

(b) barrier reef

Figure 8.10 The íormation of coral atolls
according to Darwin’s theroy of subsidence.

central

lagoon

therefore also faster in shallower water) and, not surprisingly, the rate of
growth can be decreased if photosynthesis of the zooxanthellae is reduced by
sediment-laden water or Chemicals (see Section 8.6.4). Growth rates may
also decline with age and increasing size of a colony. In general, coráis are
regarded as slow-growing, with measured rates of growth usually varying
from <1 to 10 cm yr -1 .

However, growth rates of individual coral species do not necessarily describe
the rate of growth of an entire reef system. This is partly because different

221

species of coráis have different growth rates, but also because growth and
expansión of the reef is regulated by many other factors such as predation,
competition for space with other organisms, and light intensity, to ñame only
a few. Further, the limestone framework is continually being destroyed by
biological activities and physical events (see below). Estimates of total reef
growth can be made from measured changes in reef topography over several
years, or from geological information on the thickness of reef limestone
deposits. These estimates of net vertical upward growth of reefs vary greatly,
from only a few millimetres per year, to 30 cm per 11 years under
favourable conditions.

In order to obtain better estimates of the rate at which entire reef Systems
grow, it is also necessary to know something about the factors that destroy
the reef and the rate at which the limestone is broken down. Reefs are
subject to physical erosión by wave action and currents, and tropical storms
can cause extensive damage. Reefs are also subject to continual bioerosion,
or breakdown of the calcium carbonate skeleton by reef inhabitants. Some
organisms associated with the reef remove part of the coral skeleton by
boring into the reef, using Chemical dissolution or mechanical abrasión;
these inelude certain species of algae, clams, sponges, sea urchins, and
polychaete worms. Some animáis (e.g. herbivorous limpets and snails,
parrotfish) remove pieces of the reef skeleton inadvertently during grazing.
Small coral fragments are consumed by deposit-feeders such as sea
cucumbers, and thus become further reduced in size. These destructive
activities eventually break down reef material to fine-grained carbonate sand.
Much of the fine-grained detritus is flushed away from the reef by waves
and currents, but some accumulates in pockets between coral heads.

8.6.7 Z0NATI0N PATTERNS ON REEFS

All reefs exhibit zonation patterns resulting from a combination of bottom
topography and depth, and different degrees of wave action and exposure.
The patterns differ according to locality and type of reef, with atolls having
the most complex zonation. The major divisions are illustrated in
Figure 8.11 and discussed below, but depending on locality, the zones may
be subdivided into as many as a dozen.

The reef flat (or back-reef) is located on the sheltered side of the reef,
extending outward from the shore or coastline to the reef crest. This area is
only a few centimetres to a few metres deep, and large parís may be exposed
at low tide. The width of the reef flat varies from a few tens to a few
thousands of metres. The substrate is formed of coral rock and loose sand.
Beds of seagrasses often develop in the sandy regions, and both encrusting
and filamentous benthic algae are common. Because it is so shallow, this
area experiences the widest variations in temperature and salinity, but it is
protected from the full forcé of breaking waves. The reduced water
circulation, accumulation of sediments, and periods of tidal emersión
combine to limit coral growth. Although living coráis may be scarce except
near the seaward section, this area of many microhabitats supports a great
number of species in the reef ecosystem, with molluscs, worms, and decapod
crustaceans often dominating the visible macrofauna.

The reef crest (or algal ridge) lies on the outer side of the reef, with its
exposed seaward margin marked by the line of breaking waves. As the ñame
implies, the reef crest is the highest point of the reef, and it is exposed at

222

Figure 8.11 A generalized cross section of a typical Caribbean fringing reef, illustrating the
major ecológica! zones.

low tide. The width of this zone varíes from a few to a few tens of metres.
la some localities, encrusting red coralline algae are dominant; in other
reefs, brown algae predomínate in this zone. Living coráis are very scarce
where wave action is severe; usually only one or two robust coral species
dominate in this región.

The outermost seaward slope (also called fore-reef) extends from the low
tide mark into deep water. The upper part of this zone is broken by deep
channels in the reef face, through which water surges and debris from the
coral reef leaves. Large coráis dominate here, and there are many large fish.
The máximum number of coral species tends to occur at 15-25 m, then
declines fairly rapidly with increasing depth. At 20-30 m depth, there is
little wave action and the light intensity is reduced to about 25% of that at
the surface; here, coráis tend to be smaller branched forms. At 30-40 m,
sediments accumulate on the gentle slope and coral becomes patchy in
distribution. Sponges, sea whips, sea fans, and ahermatypic coráis become
increasingly abundant and gradually replace hermatypic coráis in deeper and
darker water. At 50 m, the slope steepens into deep water. The depth limit
for reef-building coráis is about 50-60 m in the Pacific, and about 100 m in
the Caribbean; the difference is probably related to differences in light
penetration.

8.7 MANGR0VE SWAMPS

Mangrove swamps, also called mangáis, are a common feature covering
60-75% of tropical and subtropical coastlines. These forests of trees and
shrubs that are rooted in soft sediments occur in the upper intertidal zone.
They produce a marine system that is similar to a saltmarsh in having aerial
storage of plant biomass and in harbouring both terrestrial and marine
species. The euryhaline plants making up this specialized community are
tolerant of a wide range of salinities and are found both in fully saline
waters and well up into estuaries, but they are restricted to protected shores
with little wave action. The distribution of mangroves overlaps with that of

223

coral reefs, but extends farther into subtropical regions. In many areas,
mangrove swamps border coastlines protected by barrier reefs.

8.7.1 WHAT ARE MANGROVES?

The term ‘mangrove’ refers to a variety of trees and shrubs belonging to
some 12 genera and up to 60 species of flowering terrestrial plants
(angiosperms). Dominant genera inelude Rhizophora , Avicennia , and
Bruguiera. They have in common the foliowing features:

(a) They are salt-tolerant and ecologically restricted to tidal swamps.

(b) They have both aerial and shallow roots that intertwine and spread
widely over the muddy substrate in an impenetrable tangle (Colour

Píate 38). The substrate is oxygen-poor, and the aerial roots allow the plants
to obtain oxygen directly from the atmosphere. Many of the mangrove
species also have special prop roots extending down from the trunk or from
branches to serve as extra support.

(c) Mangroves have special physiological adaptations that prevent salt
from entering their tissues, or that allow them to excrete excess salt.

(d) Many mangrove plants are viviparous, producing seeds that germinate
on the tree. Young plants drop from the tree into the water, and the floating
plants are dispersed by water. The Ufe eyele of these long-lived plants is
illustrated in Figure 8.12.

Some Indo-Pacific mangrove forests may contain 30 or more species of
mangroves. There are fewer in Atlantic areas; a total of 10 species is
distributed throughout the New World, and mangrove swamps in Florida, for
example, support only three species.

Figure 8.12 The lite eyele of viviparous mangrove trees.

224

8.7.2 ECOLOGICAL FEATURES OF MANGROVE SWAMPS

The physical environment of mangáis is characterized by considerable
fluctuation in salinity and temperature. This is also a región that is strongly
influenced by tidal action. Water exchange transports nutrients into mangrove
areas, and exports material out. Tidal flow also results in an inflow and
outflow of animáis, such as fish and shrimp, into the tidal area. Animáis
living high in the intertidal zone are subjected to the greatest environmental
variation and to potential desiccation. Nevertheless, the plants and animáis
are adapted to tidally-induced fluctuation, and the largest mangrove swamps
are in areas with a large vertical tidal range.

Mangroves are found in regions of little wave action, and the intertwining
roots of the plants further reduce water velocities. This results in trapping of
suspended sediments and organic material (particularly leaves) which settle
on the bottom to form black mud. The sediments tend to be anoxic because
of high bacterial activity and because of poor circulation within the
fine-grained substrate.

There is a Progressive change from marine to terrestrial conditions from the
seaward side of a mangrove area to the landward edge. There is a
corresponding zonation of different mangrove species, based at least partly
on their respective salt tolerances.

Ecologically, a mangrove community can be divided into (a) the
above-water forest, (b) the intertidal swamp, and (c) the submerged subtidal
habitat. These distinctive zones support unique combinations of species
which are described below.

The above-tide forest formed by the trunks and leaf canopy of the
mangroves is an arboreal environment inhabited by terrestrial species. Birds,
bats, lizards, tree snakes, snails, land crabs and mangrove crabs, spiders, and
insects are all common residents, with insects being the most diverse and
most abundant. The birds and bats are mostly insectivores or are piscivorous,
feeding on small fish. The crabs are detritivores or omnivores and may feed
on marine prey during low tide. In some areas, domestic animáis (cattle,
goats, or camels) may graze on the mangrove leaves. A study of Florida
mangroves showed that about 5% of the total leaf production was consumed
by non-mammalian terrestrial grazers, the rest entering the aquatic system as
debris and becoming available for marine detritivores, either fish or
invertebrates.

The intertidal swamp offers a variety of different substrates and different
microhabitats to support a more diverse community of marine species. Some
organisms attach to the mangrove roots, others reside in or on the mudflat or
mudbanks. Barnacles and oysters are conspicuous epifauna on the roots, with
the latter often the dominant contributor to community biomass. Certain
species of isopods bore into the woody prop roots, and their activities may
sever the roots, although the total impact to the mangrove swamp is usually
limited. Periwinkles (snails) are found in abundance crawling over the roots
in the upper intertidal zone. Some polychaete worms are also associated with
the root system, with some tube-building species attaching to this hard
surface. In this area, combined densities of snails, nematodes, and
polychaetes commonly exceed 5000 m -2 .

The intertidal mudflat is the home of numerous burrowing fiddler crabs
( Uca ), and sea cucumbers commonly are present on the surface of the mud.
Both of these groups feed on detritus. Red and green benthic algae are

225

grazed by amphipods and some species of crabs. Pacific mangroves are
frequented by large-eyed mud-skippers (genus Periophthalmus ), fish that
burrow into the mud but spend much time out of water, using modified fins
to crawl on the mud fíat or, in the case of one species, to climb the
mangrove roots. Various species of shrimp and fish move in and out of this
región with the tides.

Leaf fall is a major source of nutrients and energy in the intertidal swamp,
and many of the residents are detritivores. Some remove suspended detritus
by filter feeding (e.g. oysters), others feed on organic material in the
sediments by deposit feeding (e.g. burrowing polychaetes), and others like
crabs, shrimp, and amphipods capture larger particles of debris with their
claw-like appendages. Most animáis in the community probably consume
detritus in addition to living plant and animal tissue.

The subtidal zone also has sediments of fine-grained mud with a high
organic content, and sand patches may be present as well. The subtidal
mangrove roots support a rich epiflora and epifauna of algae, sponges,
tunicates, anemones, hydroids, and bryozoans, and their crowded numbers
indicate that competition for space is intense on this substrate. In some areas,
turtle grass ( Thalassia ) may be the dominant benthic plant, and it serves to
stabilize the mud bottom. Burrowing animáis (e.g. crabs, shrimp, worms) are
common, and their burrows facilitate oxygen penetration into the mud and
thus ameliorate anoxic conditions. Fish are most common in this zone, and
many of them are plankton-feeders. The fish, as well as crabs, lobsters and
shrimp, form the basis for local fisheries.

The primary producers in this system inelude not only the mangroves
themselves, but benthic algae, seagrasses, and phytoplankton. Few
production studies have been conducted in mangroves because they are a
particularly difficult environment in which to work. However, it is clear that
mangrove swamps are rich in reeyeled nutrients. Although large quantities of
detritus may be exported from a mangrove community, the roots also trap
organic-rich detritus which is broken down and decomposed in the
sediments; the reeyeled nutrients then become available to be taken up by
the roots of the mangroves. Thus the mangrove system is not solely
dependent on nutrients dissolved in the surrounding oligotrophic seawater.
Mangroves are also located in regions of intense solar radiation, and the
combination of high nutrients plus high light should lead to high gross
primary production rates. Plant respiration is variable and is possibly related
to the degree of salinity stress in particular localities. However, it is
estimated that mangrove swamps contribute between 350 and
500 g C m“ 2 yr -1 net production to Coastal waters.

QUESTIQN 8.9 How do valúes of nel primary producüvily in mangrove
swamps compare with those íor phytoplankton production in tropical
nuiriem-deficieni oeeanie waters'? Refer to Table 5.1.

8.7.3 IMPORTANCE AND USES OF MANGROVES

Mangroves figure importantly in the livelihoods of peoples living within or
adjacent to these habitats. The trees themselves have traditionally been used
for firewood and charcoal. The timber is water-resistant, so it is also used to
construct boats and houses. The leaves are used for roof thatching and as
fodder for cattle and goats. Even cigarette wrappers are manufactured from
the young, unfolded, leaf sheaths of a certain mangrove species.

226

Most of these tropical Coastal communities have long-standing fisheries
based not only on fish like mullet, but also on the abundant populations of
shrimp, crabs, bivalves, and snails. Fish nets and traps are often constructed,
at least in part, from parts of the mangrove trees, and tannin extracted from
the mangroves is used to increase the durability of fishing nets and sails.

Mangroves also have great importance in non-commercial aspects. They
form protective barriers against wind damage and erosión in regions that are
subject to severe tropical storms. In some areas, mangroves may facilitate
the conversión of intertidal regions into semi-terrestrial habitats by trapping
and accumulating sediment. For example, mangroves have spread seawards
at rates of between 100 m and 200 m per year in Indonesia. The root system
also serves as a protective nursery ground for many species of fish, shrimps,
juvenile spiny lobsters, and crabs. The forest canopy not only supplies food
for many of the arboreal and marine inhabitants, either directly or as detritus,
but it is utilized for nesting sites for a variety of tropical birds.

8.8 DEEP-SEA EC0L0GY

The vast majority of the seafloor lies permanently submerged below tidal
levels yet, relative to the intertidal regions, comparatively little is known
about life in the bathyal, abyssal, and hadal zones (see Figure 1.1). This, of
course, is due to their relative inaccessibility. Although it is possible to dive
to several thousand metres in submersibles or to employ remote-controlled
cameras, the number of hours of direct observations in the deep sea has so
far been extremely low. Most information on deep-sea ecology comes from
indirect inferences based on animáis contained in benthic samples obtained
from ships. Whatever the method, expense is a limiting factor in deep-sea
research. Few countries or institutions have submersibles to use for basic
research, and few have large research ships equipped to obtain deep-sea
samples. Collecting a sample from 8000 m depth with towed gear, for
example, requires a very large winch with at least 11 km of cable in order to
allow for the towing angle. It may take up to 24 hours to let out that much
wire, obtain a sample, and then retrieve it. With large ship costs easily
exceeding tens of thousands of dollars per day, a single sample containing a
few benthic animáis can be beyond the budget of most oceanographic
research facilities. Compounding this problem is the fact that animal life is
just not very abundant in many deep-sea areas, so that it is desirable to have
large numbers of samples. Nonetheless, new techniques for collection and
observation, combined with accumulating numbers of analysed deep-sea
samples, permit a general assessment of benthic life in deeper water.

The deep-sea environment has been generally regarded as stable and
homogeneous in terms of many physical and Chemical parameters. Water
temperatures are generally low (from —I o to 4°C) and salinity remains at
slightly less than 35. Oxygen content is also constant and is rarely limiting,
with the exception of areas beneath upwelling zones or in some basins where
water exchange is slight (e.g. the Cariaco Trench in the Southern Caribbean
Sea). Soft bottom sediments, originating from land and/or from the sinking
of dead planktonic organisms, cover most of the deep seafloor. Hard
substrates are largely limited to mid-ocean ridges and seamounts that jut up
from the sea bottom. Relative to surface currents, bottom currents in the
deep ocean basins are slow (generally <5 cm per second) but more variable
than once believed. Some areas experience abyssal (or benthic) storms

227

lasting up to a few weeks, during which bottom currents increase in speed
and may reverse direction. Deep boundary currents that move along
continental margins may have velocities of up to 25 cm s _1 , and these may
cause sediment resuspension and thus influence sediment redistribution.
Deep-sea environments may experience seasonal variability in the amount of
organic material that sinks from the euphotic zone to the seafloor.

8.8.1 FAUNAL COMPOSITION

Most animal phyla are represented in this dark environment of low
temperatures, high pressures, and predominantly soft substrates. It has been
known since the time of the Challenger expedition that there are, however,
changes in the relative abundance of different types of zoobenthos with
increasing depth. Figure 8.13 is based on benthic samples taken from the
Kurile-Kamchatka Trench in the North Pacific in the 1950s. It shows, for
example, that sponges form a dominant component of the macrobenthic
biomass between 1000 m and 2000 m, but they are small and scarce below
2500 m. Starfish are important members of the trench community down to
7000 m, at which depth they disappear. Holothurians (sea cucumbers),
however, increase in relative abundance in deeper areas; one species (Elpidia
longicirrata ) makes up about 80% of the total biomass in the trench.

Globally, holothurians frequently domínate the biomass in depths over
4000 m where sediments are relatively rich in organic matter. Numerically,

Figure 8.13 The percentages oí different
animal groups in the biomass of macrobenthos
at different depths in the Kurile-Kamchatka
trench.

1000

2000

3000

4000

5000

^ 6000

7000

8000

9000

10000

sponges

(Poniera)

holGtburíans

(Echinodermata)

asteroids (starfish)
(Echinodermata)

■ sea anemones

(Cnidaria-Anthoza)

coráis

(C n ida ria-Ant hoza)

polychaetes

(Annelida)

echiurids and
sipuncutids

Mollusca

crustaceans

{Artbropoda}

others

biomass (per cent)

228

small burrowing polychaetes commonly make up 50-75% of the macrofauna
in many, widely scattered, soft-bottom deep-sea sites. Small crustaceans
(amphipods, isopods, tanaids) are also common deep-sea macrobenthic
species, followed by molluscs (especially clams), and a variety of worms
(sipunculids, pogonophora, echiurids, and enteropneusts). Brittle stars
(ophiuroids) can be abundant in some areas; for example, they make up over
60% by numbers of the macrobenthos in the Rockall Trough west of Ireland.

Certain groups of animáis attain their greatest abundance and diversity in the
deep sea. The soft-bodied or calcareous sponges that are common in shallow
water are largely replaced by glass sponges with siliceous spicules in deep *
water. Cnidaria are principally represented in the deep sea by sea anemones
that live in burrows, and by sea pens and gorgonian coráis that may form
densely populated beds under eutrophic waters where there is sufficient
suspended material for feeding. Slender, branching colonies of black coráis
have been found in the greatest depths. The Pogonophora are mostly a
deep-water group found down to 10000 m, and echiurid worms become
more common in depths exceeding 5000 m. Some echiurids reach body
lengths of 1 m, and they can occur in dense aggregations in organically-rich
sediments where they form a large proportion of the biomass. The more
primitive crinoids, the stalked sea-lilies, are mostly restricted to deep-sea
habitats.

Benthic foraminiferans and related protozoans, the giant xenophyophores
(see Section 7.2.1), increase in importance in deep water, both in terms of
abundance and biomass. Unlike the shallow-living species with calcareous
tests, deep-water foraminiferans have proteinaceous tests or exoskeletons
made of agglutinated sediment particles. In certain areas, 30-50% of the
seafloor may be covered by foram pseudopodia and, in the Aleutian Trench,
forams comprise 41% of the meiofauna. Xenophyophores are known to
occur in nearly all areas of the deep-sea basins at depths below 1 km. They
may occur in densities of up to 20 m -2 , and they constitute up to 97% of the
total benthic biomass in some areas of the South Pacific.

Some animal groups show a tendency toward gigantism in the deep sea (see
also Section 4.4); these inelude the benthic foraminifera and the
xenophyophores, as well as certain amphipod species that attain lengths of
about 28 cm. However, there is a reverse tendency in some groups toward
miniaturization, and the deep-sea meiofauna is numerically dominant over
the macrobenthos. Nematodes are ubiquitous in marine soft substrates and
make up 85-96% of the deep-sea meiofauna. Harpacticoid copepods and
ostracods are also common deep-sea members of this size category, the
former group constituting 2-3% of the meiofauna in abyssal zones. Tanaids
are extremely diverse in the deep sea, and many of the species are
meiofaunal; in the north-west Atlantic, they occur in densities of
about 500 m -2 .

Certain animal groups are poorly represented in deep water. Decapod
crustaceans (e.g. crabs, shrimp, lobsters), sea anemones, and echinoid
echinoderms are absent or uncommon below about 6000 m. Fish are also
rare in very deep waters; one of the deepest captured fish carne from 7230 m
in the Kurile-Kamchatka Trench. These generalizations are largely based on
collections made with dredges or trawls, both of which are difficult to use
over rocky substrates or in relatively steep-walled trenches, and both of
which can be avoided by swimming animáis. It is well to keep in mind that
Jacques Piccard and Lieutenant Don Walsh, who together made the deepest

229

dive in a bathyscaphe, observed flatfish and shrimp at over 10000 m; neither
of these groups of animáis have been collected by conventional gear from
such great depths.

CUESTION 8.10 Whai explanatíons can yon give for why large sponges are so
sueeesst'u) in shallower areas, and why sea cucumbers o fien domínate the
macrobenthos of deep water?

Some deep-sea residents have a cosmopolitan distribution and are found in
all the major oceans; other species are restricted to relatively small areas. In
general, species become more limited in geographic range as water depth
increases. Only about 20% of the species present below 2000 m in the
Atlantic Ocean are also found in the Pacific or Indian oceans.

Table 8.1 Percentages of species living below 6000 m depth that are endemic to
the hadal región.

Taxonomic

group

Number of
hadal species

% endemic

Foraminiferans

128

Sponges

Cnidaria

Polychaetes

Echiurid worms

Sipunculid worms

Crustaceans

bamacles

cumaceans

100

tanaids

isopods

amphipods

Molluscs

aplacophorans

snails

bivalves

Echinoderms

crinoids

holothurians

starfish

brittlestars

Pogonophorans

Fish

Many species found in areas deeper than 6000 m are endemic to the hadal
región, and many are restricted to a particular trench. Table 8.1 lists the
number of hadal species known in particular invertebrate groups, and the
percentages of these that are endemic to this deep-sea región. Endemic
species constitute up to 75% of the benthos in certain Pacific trenches. The
high degree of endemicity suggests that trenches are fundamentally isolated
habitats which are centres for the generation of new species.

Trenches often have relatively high abundances of aplacophorans (wormlike,
shell-less molluscs), enteropneust worms, and echiurid worms, all of which
are poorly represented elsewhere. In the Aleudan Trench at depths of
7000-7500 m, the macrofauna is dominated by polychaetes (49%), bivalves
(12%), aplacophorans (11%), enteropneusts (8%), and echiurid worms (3%).

230

The meiofauna is dominated by benthic foraminifera (41%), followed by
nematodes (36%) and harpacticoid copepods (15%). Some of the zoobenthos
of trenches exhibit palé coloration and are blind, characteristics that are
shared with cave fauna. Very large size is also a characteristic of some hadal
isopods, tanaids, and mysids.

Deposit-feeding infaunal animáis are dominant in the soft, organically-rich
sediments of the deep sea, usually comprising 80% or more of the fauna by
numbers. At one site in the Atlantic, at a depth of 2900 m, 60% of the
polychaetes, >90% of the tanaids, 90% of the isopods, >50% of the
amphipods, and 45% of the bivalves are deposit feeders. Other groups, like
sea cucumbers and sipunculids, also ingest the detritus and small organisms
contained in surface or subsurface sediments. Because bottom currents are
usually slow and do not disturb compacted sediments, the topographic
features produced by these animáis persist for long periods. Faecal mounds,
burrows, trails, and tubes are some of the biological features that are
commonly recorded in the deep sea by remóte cameras.

Animáis that feed on suspended partióles are also found in the deep sea, but
they are much less abundant and are usually restricted to particular localities.
This is because most of these epifaunal animáis require relatively firm
substrates for attachment as well as high concentrations of suspended food
partióles. As a result, many types of epifaunal suspensión feeders (e.g.
common sponges, sea anemones, barnacles, mussels) show a marked
decrease in abundance with increasing depth and distance from shore. They
do flourish, however, on the rock substrates found on mid-ocean ridges and
seamounts, and some may dominate in deep-sea sulphide communities (see
Section 8.9). Relative proportions of deposit feeders and suspensión feeders
vary throughout the deep sea according to the degree of organic enrichment
of the sediments and the supply of suspended food.

It is useful, in discussing deep-sea distribution of epifauna, to distinguish
between two types of suspensión feeding. Active suspensión feeders (e.g.
shallow species of sponges and tunicates) use their own energy to pump
water through a filtering structure. These animáis are successful in
environments where suspended particle concentrations are high enough to
repay the energetic costs of pumping. Passive suspensión feeders (e.g.
crinoids, some polychaetes, and most benthic Cnidaria like sea anemones
and sea-fans) rely on external water currents to convey food to feeding
appendages that are held into the flow. The passive feeders succeed only in
environments where flow conditions are predictable and fast enough to
supply them with sufficient particulate food. Suspended particle
concentrations decrease with depth, but flow conditions become more
predictable. Active suspensión feeders disappear as suspended loads
diminish, and the large filter feeders in the deep sea are passive feeders.
Depending on the current speed, the friction of water moving over the
seafloor may create physical mixing of the bottom water; this benthic
boundary layer extends from 10 to several 100 metres above the bottom.
Turbulence in this layer can result in resuspension of bottom sediments;
heavy inorganic partióles remain cióse to the seafloor, but suspended light
organic partióles will reach máximum concentrations some distance above
the bottom. Typical deep-sea passive suspensión feeders, such as sea-lilies
(stalked crinoids) and bryozoan colonies, are found in highest densities
where there is modérate current flow and resuspension of sediments. In
contrast to their shallow-dwelling relatives, these animáis are often supported

231

by long stalks which hold them well above the seafloor where concentrations
of suspended organic material may be optimal.

Some deep-sea groups have very different feeding mechanisms from those
used by shallow-water related species. Glass sponges, for example, have an
extremely porous body wall and water currents can enter passively as well as
by active pumping. Members of one family (Cladorhizidae) of small-sized
sponges occur in deep water to about 9000 m depth; at least some of them
are highly modified carnivores that passively capture small swimming prey
by means of filaments provided with hook-shaped spicules. Whereas
shallow-water tunicates are active suspensión feeders and many are colonial
species, deep-sea representatives tend to be solitary forms that may
supplement active pumping with mucous nets held into currents to capture
food. Some deep-sea tunicates are even more highly modified and have
become carnivorous.

In addition to deposit feeders and suspensión feeders, the deep-sea food
chain ineludes many scavengers. Cameras have recorded the speed with
which a variety of swimming animáis approach bait placed on the seafloor in
deep water. These inelude giant amphipods, isopods, fish, and shrimp. Brittle
stars and some polychaetes are among the slower-moving scavengers. Strict
predators appear to be rare in very deep waters. However, diets of deep-sea
benthic animáis are not well known; feeding type is usually inferred from
anatomical structure and gut contents.

8.8.2 SPECIES DIVERSITY

The number of species of many types of macrobenthos (e.g. snails, clams,
polychaete worms) and fish tends to increase with depth from about 200 m
to 2000 or 2500 m, then declines rapidly with further depth. Based on these
observations, it was believed for many years that deep-sea species diversity
was low compared with that of shallow-water communities. However, the
development and use of a new collection device, called an epibenthic sled,
changed this perception. The epibenthic sled (Figure 8.14) was designed to

Figure 8.14 An epibenthic sled designed to collect animáis living on or just above the
seafloor. The mesh-size is small enough to retain meiofauna, and the sampler can be closed
during retrieval so that the entire sample is retained.

232

collect and retain smaller animáis, in a size category that was previously not
well sampled. When this gear was first employed in the 1960s, a single
collection sometimes contained more animáis than were collected by all the
combined expeditions of the previous 100 years. To further demónstrate the
effectiveness of this apparatus, one paper reported that over 120 new species
of cumaceans (small crustaceans) had been collected. It soon became evident
that the diversity of many smaller organisms increases with depth. For
example, the number of species of meiobenthic copepods increases to at
least 3000 m, and máximum diversity of benthic foraminiferans is found in
depths exceeding 4000 m.

It is now established that there is high species diversity in the deep sea,
especially among the small infaunal deposit feeders. As additional samples
are obtained from the deep sea and more new species are described, the
more diversity in this area seems to approach that of highly diverse
terrestrial environments, such as the tropical rain forest. Some researchers
estimate that there may be more than one million species of marine benthic
animáis, most of them living in deep-sea sediments. Species diversity does,
however, vary in different oceanic areas. For example, in the North Atlantic,
species diversity declines from the tropics toward north polar regions; but in
the Southern Hemisphere, zoobenthos species diversity in the Weddell Sea
(Atlantic sector of the Antarctic) is of the level normally associated with
tropical regions. Deep-sea diversity also may vary according to different
levels of surface primary production. In some areas, zoobenthos diversity is
depressed under areas of upwelling and high surface productivity, probably
as the result of reduced oxygen concentrations from decomposition of large
amounts of organic material.

As more areas of the deep sea are surveyed with increasingly sophisticated
gear, it is becoming apparent that the environment itself, in terms of
substrate features and/or current regime, is more diverse than was once
thought. Environmental diversity in the form of microhabitats (small areas
having slightly different environmental characteristics) can itself lead to
higher diversity in animáis. Indeed, the deep-sea benthos is patchily
distributed, with significant aggregations of animáis having been detected in
different taxonomic groups on scales ranging from centimetres to metres to
kilometres. This patchy distribution underscores the importance of obtaining
representative samples when assessing biomass and species diversity of
deep-sea animáis.

8.8.3 BIOMASS

Although the number of species is high in the deep sea, communities
occupying the typical soft-sediment seafloor are characterized by low
population densities and low biomass. Numbers of benthic individuáis (both
macrofauna and meiofauna) per unit area tend to decrease roughly
exponentially with increasing depth and, to a lesser degree, with distance
from shore. Under the central oceanic gyres, the total density of macrofauna
ranges from 30 to 200 individuáis per m 2 . With a few exceptions, the
dominant infaunal species tend to be small as well as sparse. In the central
North Pacific, meiofauna and microfauna dominate the benthos in numbers
(0.3% and 99.7%, respectively), and in biomass (63.8% and 34.9%,
respectively). Deep-sea biomass valúes do not inelude the larger demersal
species, which are more difficult to capture and to quantify on an areal basis;
their inclusión would undoubtedly increase the biomass valúes given here.

233

Table 8.2 Average biomass valúes of benthic animáis at different depths.

Depth range (m)

Mean biomass
(g wet weight m -2 )

Intertidal

3 x 10 3

to 200

200

500-1000

<40

1000-1500

<25

1500-2500

<20

2500-4000

4000-5000

5000-7000

<0.3

7000-9000

<0.03

>9000

<0.01

Average biomass valúes for different depth zones are given in Table 8.2.
Benthic biomass is highest in shallow Coastal areas within the euphotic zone,
and it is lowest under oligotrophic, central, oceanic regions. Keeping in mind
that the average depth of the world ocean is 3800 m, most of the seafloor
supports less than 5.0 g wet weight m -2 of living organisms.

However, benthic biomass at any particular depth varíes according to the
amount of organic material delivered to the seabed. This is reflected, for
example, in the different levels of biomass found in various trenches, all of
which are deeper than 6000 m. Trenches are located in regions of frequent
seismic activity, and thus are subject to brief episodes of high sedimentation
caused by slumping of sediments down the trench walls. This results in the
deposition of organically rich sediments from shallower depths, but also in
the burial of benthic communities. The biomass of hadal fauna may be very
high in trenches that lie near large land masses as they receive land-derived
sediments and organic matter, as well as organic material sinking from the
overlying nutrient-enriched and highly productive surface water. Under these
conditions, benthic biomass may range from 2 to 9 g wet weight m -2 at
depths of 6000-7000 m in the Kurile-Kamchatka Trench (North Pacific) and
in the South Sandwich Trench (South Pacific). Trenches far from land
masses and under oligotrophic water (e.g. Mariana Trench) have very low
biomass valúes of about 0.008 g m~ 2 .

Benthic productivity cannot be assessed directly from biomass valúes, but
many deep-sea species grow relatively slowly and their small biomass must
indicate low productivity. Various estimates suggest that annual secondary
production over most of the deep ocean floor is between 0.005 and
0.05 g C m -2 .

OUESTIÜN 8.1! How does the decline in benthic biomass vviih depth compare
wiih the vertical distribotíon of zooplanklon biomass? Refer to Sectioti 4.4
and Figure 4.14,

8.8.4 FOOD SOURCES

Except for localized chemosynthetic production around deep-sea hot springs
(see Section 8.9), there is no primary production in the dark, deep areas of
the sea. Food availability, not low temperature ñor high pressure, limits
benthic biomass in the deep sea. The deep-sea food chain is dependent on
surface production, and only a small percentage (1-5%) of the food
produced in the euphotic zone is transferred to the abyssal seafloor. The

234

percentage diminishes with increasing depth because of the increasing
probability that organic partióles sinking from the euphotic zone will be
consumed or will decay before reaching the bottom.

A variety of potential food sources sink from the productive surface zone
and thus may become available to deep-sea benthos (Figure 8.15). The
relative contribution of each of these sources depends on their sinking rates
and attrition in intermedíate depths.

1. Dead phytoplankton, zooplankton, fish, and mammals. In many areas
of the ocean, much of the phytoplankton is consumed by herbivorous
zooplankton in the euphotic zone. That fraction which is not eaten sinks very
slowly because of its small size and, in very deep areas, it is lost through
predation, disintegration, or decomposition at intermedíate water depths. In
some regions like the North Atlantic, however, the phasing between seasonal
phytoplankton blooms and zooplankton growth is such that much of the
phytoplankton dies without being consumed (see Section 3.6), and the
sinking phytodetritus may reach the seabed at depths down to 4000 m. The
fate of most uneaten epipelagic zooplankton is similar, although sinking rates
are somewhat faster because of their larger size. The corpses of large fish,
squid, or marine mammals sink rapidly enough so that they may reach the
seafloor in deep waters, where they become available to benthic scavengers;
however, some are no doubt consumed by large animáis at intermedíate
depths. In any event, the arrival of large animal carcasses on the sea bottom
is generally an unpredictable and rare event, except perhaps under the

235

seasonal migration routes of some fish and mammals. This is not the case,
however, in heavily fished regions where tonnes of unwanted, incidentally
captured fish (‘trash’ fish) are dumped back into the sea. The dumping of
unmarketable by-catch may amount to a very significant fraction of the total
reported fish catch, but it will only locally increase benthic food supplies.

2. Faecal pellets and crustacean moults. Compact faecal pellets of some
zooplankton (about 100-300 fim in size) are collected in mid-water traps
designed to capture sedimenting particles, and their settling rates are such
that they may reach the seafloor relatively intact. Faecal material of fish may
also reach the seabed almost undegraded. Although some animáis ingest
faecal pellets, these wastes generally contain large fractions of indigestible
materials. Moulted exoskeletons of planktonic crustaceans occur in the
benthic boundary layer, but they may result from deep-water species. Moults
are also low in nutritional valué as they are composed primarily of chitin
which cannot be digested directly by most animáis, but it is broken down by
chitinoclastic bacteria living in the guts of many species. Faecal pellets and
moults are colonized by bacteria during their descent in the water column,
and they are eventually converted to bacterial biomass. Bacteria are
important nutritional intermediates in the food cycle of the deep sea, and
they comprise a major food source for benthic deposit feeders. There is, in
fact, an increase in bacteria in the bottom sediments of the deep sea, with
numbers exceeding one million per gramme of sediment between depths of
4000 and 10000 m.

3. Macrophyte detritus. A certain amount of organic material enters the
sea near Coastal zones in the form of wood from terrestrial plants, or from
dislodged seagrasses and kelp. Some of this is carried well offshore in
currents before becoming waterlogged and sinking. Sargassum is also a
potential source of organic material in areas where it occurs (see

Section 4.4). Larger plant particles sink rapidly enough to reach the seafloor
more or less intact. Panels of wood placed at a depth of 1830 m and
recovered 104 days later were riddled with burrows made by wood-boring
clams, some of the very few animáis that can utilize wood for food. These
bivalves convert woody plant material to foods that are available to other
animáis. They produce faecal pellets that can be consumed by detritivores;
those larval or adult clams that become exposed by the disintegration of the
wood can be eaten by predators; and their dead remains become available
for scavengers. Bacteria decompose and convert other types of macrophyte
detritus into biomass available to benthic animáis.

4. Animal migrations. The vertical migrations of zooplankton and fish
result in a downward transfer of organic materials. Food that is captured in
shallower depths is converted to animal biomass that may be consumed by
predators at deeper levels, and faecal pellets may be released by migrators
when they return to deeper water (see Section 4.5). Some deep-sea fish (e.g.
angler-fish) spend their larval stages near the surface and then migrate intó
the depths as juveniles or adults, where they become potential food for
deep-sea predators. All of these events accelerate the pace at which food
enters the deep sea.

There is seasonal variability in the amount of organic material reaching the
seafloor in températe and high latitudes due to seasonal differences in
surface production. The sinking of large amounts of macrophyte detritus also
may be linked to seasonal storms that dislodge seagrasses or trees. The

236

dumping of trash fish also is largely restricted to seasons of relatively calm
weather and availability of fish schools. Some fish die after spawning (e.g.
mesopelagic blue whiting), and their carcasses deliver a seasonal signal to
the underlying deep-sea fauna.

Although some deep-sea species have seasonal reproduction, and growth
bands in mollusc shells and echinoderm skeletal plates reveal seasonal
growth in certain species, many deep-sea animáis have continuous
reproduction and their production does not appear to be linked to seasonal
surface events. Secondary production may be linked, however, to the
sporadic nonseasonal occurrence of an adequate food supply.

Although life in most of the deep sea is dependent on surface production, the
deep-sea environment is spatially and temporally separated from the euphotic
zone. In general, it is estimated that 75-95% of the organic matter in
partióles sinking from the euphotic zone is decomposed and recycled in the
upper 500-1000 m of the water column, above the permanent thermocline.

In the Sargasso Sea, surface production is >100 mg C m~ 2 day -1 and the
flux to the bottom at over 3000 m varíes from 18 to 60 mg m -2 day -1
depending on season. However, organic matter constitutes only about 5% of
the total sedimenting material, the remainder being mostly inorganic
carbonate and silicate. In the north-east Atlantic, seasonally deposited
phytodetritus has an even lower organic carbón content of less than 1.5%;
however, deep-sea animáis have been observed to feed on this material
despite its low nutrí ti ve valué. In general, only about 5-10% of organic
matter produced in the euphotic zone will reach depths of 2000 to 3000 m,
and progressively less in abyssal and hadal zones. Thus food is very scarce
in the deep sea compared with other ocean regions. Food limitation is one of
the most important factors goveming biological processes and community
structure of the deep-sea benthos.

CUESTION 8.12 Using the máximum sinking rales for diaioms (from
Secíion 3.1.1), how long would il take a single dead Chaeíocews cefl to
sink to 5700 m depth?

8.8.5 BATES 0F BIOLOGICAL PROCESSES

Accumulating evidence suggests that various biological processes in
deep-sea animáis, such as metabolism, growth, maturation and population
increase, are slow in comparison to such processes in shallow-water
environments. One of the first pieces of evidence resulted from an accident
at sea. In 1968, the research submersible Alvin slipped from its launching
eradle after the pilot and scientists were on board, but before the ports were
secured. All three men managed to exit safely, but the Alvin sank in 1540 m
of water and carried their packed lunches with it. The submersible was not
recovered until over 10 months later, at which time it was discovered that
the scientists’ lunches, although waterlogged, were still in good condition
and edible. Placed for three weeks in a refrigerator at 3°C (the same
temperature as at 1540 m), the food spoiled. This unexpected observation
stimulated experiments in which organic substrates were exposed in situ at
depths down to 5300 m; when these were recovered, they confirmed very
low rates of bacterial metabolism in the abyss. The metabolic rate of abyssal
bacteria living in sediments is from 10 to over 100 times slower than that of
equivalent bacterial densities maintained in the dark, at the same low
temperature, but at atmospheric pressure. Bacterial productivity is thought to

237

range from about 0.2 g C m 3 day 1 (at 1000 m) to 0.002 g C m 3 day 1
(at 5500 m).

Low metabolic rates have also been reported for some benthopelagic
animáis, including teleost fishes. However, studies of large deep-sea
epibenthic decapods and echinoderms indícate respiration rates comparable
to those of related forms in shallow water when measured at the same
temperature. When oxygen uptake by benthic communities is compared, that
in the deep sea is two to three orders of magnitude lower than that of
shallow-water communities. This reduction in oxygen uptake is partly due to
a lower density of organisms per unit area, but it also reflects the lower
metabolic activity of deep-sea organisms.

Another indication of slower biological processes carne from studies of
Tindaria callistiformis , a small (<9 mm long) clam inhabiting soft sediments
of the North Atlantic at 3800 m depth. Radioactive dating of the shells of
this little deposit feeder suggested that it grows extremely slowly, with the
shell increasing in length at about 0.084 mm yr -1 (see Figure 8.16 for a
comparison with other molluscs). This would mean that Tindaria requires 50
years to reach sexual maturity, and that its life span would be about 100
years. However, the technique used to obtain these results has since been
criticized, and faster growth rates have been estimated for other deep-sea
benthos.

The wood-boring clams (see Section 8.8.4) that rely on ephemeral falls of
wood in the deep sea are notable exceptions to slow growth rates. In order to
search out and colonize new sources of wood, they have evolved
opportunistic life strategies that involve rapid growth, early maturity, and
production of many young.

QUESTION 8.13 Would you eonsider Tindaria caüistijonms and wood-boring
clams to be priman ly exampies of r- or /¡f-seleeted s pee Íes? (Reler lo
Section 1.3.1 and Table Lid

Many deep-sea species have low fecundity. The number of eggs produced
per individual is generally much lower in many deep-sea residents when
compared with their shallow-water relatives; this can be related to the
miniaturization that occurs in many groups. One small (< 1 mm long)
deep-sea clam, Microgloma, produces only two eggs at a time, and even
larger deep-sea clams may produce only a few hundred eggs at any one
time. In contrast, shallow-water clams typically spawn tens or hundreds of
thousands of eggs. Although type of development is not known for many
deep-water species, the dominant mode appears to be production of
lecithotrophic larvae.

Low fecundity and therefore low dispersal suggest slow rates of
recolonization in the deep sea, and this has been confirmed by experimental
studies. Boxes of sterilized azoic sediment were placed at depths of 10 m
and 1760 m, and examined after 2 months and after 26 months. The shallow
boxes were colonized rapidly by invertebrates; after only 2 months, the
boxes contained 47 species and 704 individuáis (35 714 individuáis m -2 ). In
comparison, the deep-sea boxes yielded only 14 species and 43 individuáis
(160 individuáis m“ 2 ) in the same period of time. Even after 26 months,
recolonization of the deep-sea boxes was such that they contained 10 times
fewer individuáis and species than samples taken from the surrounding
sediment at the same depth.

238

The low rates of metabolism and production shown by many deep-sea
animáis can be correlated with low food supply in their environment, but it
is also likely that the food requirements of deep-sea species are much lower
than those of surface-dwelling animáis. Only a limited number of
biochemical studies have been done on deep-sea animáis, but these suggest
that body protein concentrations and caloric content decrease with depth in
fish and crustaceans. On a weight for weight basis, the food requirements of
a slow-moving rat-tail fish living in the deep sea are likely to be about 20
times less than those of an active epipelagic salmón. Low rates of
metabolism may also result from the physical-chemical constraints on
enzyme kinetics that are known to occur at high pressures and low
temperature.

8.8.6 FUTURE PROSPECTS

Much remains to be explored in the deep sea. Only a very small percentage
of the seafloor has been examined using dredges, cores, submersibles, or
remóte cameras. By 1995, less than 100 m 2 of the deep seafloor had been
quantitatively sampled. Certainly we can expect that many more new species
will eventually be discovered. More detailed physical studies, particularly of
water flow over the seafloor, may further alter our perception of
homogeneity in this environment. For example, the effect of abyssal storms
on benthos is not known.

We need to learn more about the biology of individual species in terms of
their food and feeding patterns and energy requirements; their patterns of
reproduction, development, and growth; and their interactions with other
species. Only then can we approach an understanding of the factors that
establish community structure, that determine production, and that maintain
high species diversity in the deep sea.

8.9 HYDROTHERMAL VENTS AND COLO SEEPS

In 1977, scientists working off the Galápagos Islands discovered unusually
high seawater temperatures at about 2500 m depth in an area where new
seafloor is being formed. The hydrothermal activity accompanying this
process manifests itself in the release of mineral-laden fluid either emitted as
a warm (5-100°C) diffuse flow from cracks and crevices in the seafloor, or
emerging as plumes of superheated (250-400°C) water from chimneylike
vents. As the hydrothermal fluid mixes with the surrounding seawater,
temperatures are moderated to between 8 o and 23°C. Chemical analyses of
the water in the vent areas revealed low concentrations of oxygen but very
high concentrations of hydrogen sulphide (H 2 S), a compound that is usually
highly toxic to animáis even in much lower concentrations.

QUESTIQN 8.14 What is the normal ambieni seawater temperature al u depth
of 2500 m? Refer to Seelion 2.2.2.

One of the most exciting events in benthic marine ecology occurred when
scientists, diving in a submersible, discovered extremely dense concentrations
of benthic animáis living in this hydrothermal vent area. Since that time,
similar deep-sea communities of animáis have been found in other localities
around the world, all of them in areas of tectonic activity. Sites at which

239

biological investigations have been conducted inelude the Mid-Atlantic
Ridge and spreading centres along the rim of the Pacific Ocean basin.

8.9.1 CHEMOSYNTHETIC PRODUCTION

Many of the animáis in the densely populated vent communities are of
extraordinarily large size. The occurrence of very high benthic biomass in
deep waters, far removed from surface photosynthetic production,
immediately raised the question of how these animáis obtained sufficient
food.

Further studies revealed the existence of a food chain driven entirely by
geothermal (terrestrial) energy, and not dependent on solar energy. Vent
communities are dependent on the presence of hydrogen sulphide, a reduced
sulphur compound that is released in hydrothermal fluid. This compound is
utilized by sulphur-oxidizing bacteria (e.g. Thiomicrospira and Beggiatoa ),
and the energy released by the oxidation is used to form organic matter from
carbón dioxide by the same biochemical pathway that is employed by
photosynthetic organisms. The reaction requires molecular oxygen which is
provided by the surrounding seawater. The biochemistry of the
chemosynthetic production can be generally summarized as:

co 2 + h 2 s + o 2 + h 2 o -► ch 2 o + h 2 so 4

carbohydrate

In vent communities, chemosynthetic bacteria are the primary producers of
the food chain, and the bacterial biomass becomes available for consumption
by higher animáis. Mats of filamentous bacteria (up to 3 cm thick at some
sites) can be grazed by animáis like limpets, and bacteria suspended in water
can be filtered by suspensión feeders. In some cases, the bacterial production
proceeds within tissues of host ánimals iri special symbiotic relationships.

At present, the sulphur-oxidizing bacteria have received the most attention
and are believed to comprise most of the bacterial biomass in vent areas.
However, it is probable that other types of bacteria, utilizing different
reduced materials (e.g. methane, ammonia) as sources of energy, also
contribute to chemosynthetic production in these regions. In any event,
bacterial production at hydrothermal vents is estimated to be two to three
times that of photosynthetic production in the overlying water.

8.9.2 VENT FAUNA

Approximately 95% of the animáis discovered at hydrothermal vents have
been previously unknown species. To date, about 375 new species have been
described, many requiring the establishment of new taxonomic families
because they are so different from related species.

Spectacular giant, red, tube-dwelling worms found initially at the Galápagos
vents proved to be a new genus and species, Riftia pachyptila (Colour
Píate 39). These vestimentiferans (see Section 7.2.1 and Table 7.1) are
encased in leathery tubes, with only a plume of many tentacle-like
respiratory filaments protruding from the open end. They are highly unusual
in lacking a mouth or digestive tract, but they are free-living and not
parasitic. The largest vestimentiferans from the Galápagos site measure
1.5 m long and 37 mm in diameter, and have tubes of up to 3 m in length.
These animáis have extraordinarily high growth rates of up to 85 cm year -1 .

Densities of Riftia can be as high as 176 individuáis m -2 , and biomass of
Riftia alone ranges from 6800 to 9100 g wet weight m -2 . When combined
with the wet weight of large bivalves living in the same site, biomass of this
particular vent community can exceed 20-30 kg m -2 .

QU ESI ION 8,15 How does the biomass of i he Galápagos vent community
compare with typical biomass valúes at 2500 m deptli? Refer to Tabie 8.2.

Riftia has a special internal organ known as a trophosome (meaning ‘feeding
body’), which contains masses of symbiotic bacteria. These bacteria make up
to 60% of the dry weight of a Riftia individual, and it becomes a semantic
question as to whether this organism is more a bacterial colony than a worm.
The haemoglobin of Riftia is unique in being able to carry both oxygen and
hydrogen sulphide simultaneously. The bacteria obtain energy from
hydrogen sulphide brought to them in the blood system of the worm; the
bacteria utilize CO 2 and the energy derived from the oxidation of the
sulphide to form organic carbón. Some of this organic carbón is, in some
way, passed into the tissues of the worm. Whether this is the solé source of
nutrition for these large worms, or whether they are also able to absorb
dissolved organic matter (e.g. amino acids) from the surrounding seawater,
remains unanswered.

Another conspicuous and dominant animal at the Galápagos hydrothermal
vents is a clam, Calyptogena magnifica , that reaches lengths of 30-40 cm
(Colour Píate 40). The soft body parts of this bivalve are red, as they are in
Riftia. In both animáis, the colour is derived from haemoglobin in the blood.
Most molluscs contain the blood pigment haemocyanin; its replacement in
Calyptogena by haemoglobin, a more efficient oxygen-carrier, may be an
adaptation to the low and variable oxygen concentrations in the surrounding
water. The gills of Calyptogena contain masses of attached sulphur bacteria
and the clams, like Riftia , benefit nutritionally from this symbiotic
relationship.

Growth rates of Calyptogena have been calculated to be from 10 mm yr -1
to as high as 60 mm yr -1 ; these rates are compared with growth rates of
various other species of molluscs from different localities in Figure 8.16.
Note that clams and mussels from vent areas have rates of growth that are
comparable to those of shallow-water relatives, but they are approximately
three orders of magnitude greater than those estimated for another deep-sea
clam, Tindaria (see Section 8.8.5). Metabolic rates of Calyptogena and other

Figure 8.16 Growth curves for different species of shallow- and deep-water clams and

mussels.

California Mussels
Mytilus californianus
West Coast, U.S.A.
SUBLITT0RAL

Giant Vent Clams
Calyptogena magnifica
_ (2600 m)

Galápagos Vent Mussels
Bathymodiofus thermophilus

(2500 m) _ At | antjc Ribbed Mussels
Geukensia demissa
Virginia, U.S.A.

INTERTIDAL

Blue Mussels
Mytilus eduiis
England

HIGH LITT0RAL
lili

Deep-sea Clams
Tindaria callistiformis
(3800 m)

v lOOyears

I I l I l l I I I

241

large vent animáis are also similar to those of shallow-water relatives, and
are orders of magnitude higher than those of related animáis in other parts of
the deep sea.

Related species of vestimentiferans and large clams similar to Calyptogena
magnifica occur at other hydrothermal vent sites. Other vent molluscs
inelude Bathymodiolus thermophilus , a giant mussel that has symbiotic
bacteria on its gills but also is capable of suspensión feeding, and limpets
(>30 species) and snails which graze directly on mats of free-living bacteria
covering hard surfaces. Several types of suspension-feeding polychaete
worms are associated with vents, including tube worms (Family Alvinellidae)
that attach in large numbers to vents emitting superheated water.

Deposit-feeding polychaetes are found in sediments around the vents. Long,
thin enteropneusts, commonly known as spaghetti worms because of their
appearance, can be abundant. Various types of crabs are found at most vents;
some are scavengers, at least part of the time, and some prey on Riftia, small
mussels, or polychaetes. Shrimp in densities of up to 1500 m -2 surround
smoking vents on the Mid-Atlantic Ridge, and they apparently feed on
bacterial mats. Sea anemones are abundant at certain sites, but other types of
cnidarians are absent in these communities. A primitive type of barnacle
dominates some Pacific hydrothermal localities. Fish are not usually
important members of these communities; only five species ha ve so far been
recorded from vents.

Zooplankton are found in higher densities around vents than in surrounding
waters. Copepods, amphipods and other planktonic crustaceans have been
described, but few have been studied in detail. A new copepod genus
(Stygiopontius ), with seventeen species, occurs at every vent site. Vent
meiofauna is dominated by nematodes and benthic copepods, as it is in other
deep-sea regions. As more hydrothermal vents are discovered and sampled,
the list of new species of animáis grows rapidly.

8.9.3 SHALLOW VENTS AND COLO SEEPS

All of the explored hydrothermal vent sites are in waters deeper than
1500 m, but geothermally-driven chemosynthetic production is not restricted
to the deep sea. There are, for example, hydrothermal vents that release high
concentrations of sulphides in intertidal areas off Southern California. There,
benthic mats of sulphide-oxidizing bacteria contribute to the total primary
production of the area along with photosynthetic production by benthic
plants and phytoplankton. Limpets living near these vents are reported to
graze on the bacterial mats, whereas limpets living in non-vent areas
typically graze on photosynthetic algae encrusting rocks.

In 1984 a community of exotic organisms was discovered in the Gulf of
México, at the base of the Florida Escarpment. This massive limestone cliff
rises some 2000 m above the sea bottom; at a depth of 3270 m, hypersaline
waters containing high concentrations of sulphides and methane seep out
onto the seafloor. Although the water temperature is low, the organisms in
this coid sulphide seep area are remarkably like those found in hydrothermal
vents. White bacterial mats cover exposed substrates. There are dense
concentrations of 1-m-long tube worms (a new genus and species of
vestimentiferan) as well as thick patches of large mussels and clams (the
latter a new species of Calyptogena ). Snails, limpets, and crabs are also
conspicuous inhabitants of this particular seep.

242

Cold-water seeps result from a variety of causes, and they have been found
along continental margins and in subduction zones where oceanic crust is
carried back down into the Earth’s mantle. Those that have been explored
support similar assemblages of animáis. Seep communities are also
dependent upon chemosynthetic production by sulphide bacteria, and not on
photosynthesis. The discovery of coid seeps demonstrates that the most
important component necessary for high biological production in the deep
sea is a source of reduced inorganic compounds, not heat. The existence of
hydrothermal venís and coid seeps in deep waters indicates that low
temperature and high pressure do not limit activities of deep-sea organisms.
In deep areas where benthic ecology is dependent on photosynthetic
production at the surface, biological processes and benthic production are
limited by low food. In areas where sulphide-based food chains are possible,
biological production in the deep sea may exceed that in the euphotic zone.

Although each hot vent or coid seep site has distinctive physical features and
distinctive fauna, the communities of animáis associated with high sulphide
concentrations are similar in some respects. The dominant species are often
ecologically similar, if not taxonomically related. Large vestimentiferans
commonly occur at many sites, as well as similar species of limpets, clams,
mussels, and crabs. All of the communities are characterized by having high
population densities, high biomass, and rapid growth rates. These are unique
concentrations of life in depths that usually are characterized by low density
and low productivity; as such, these sulphide communities are appropriately
referred to as ‘oases\ Bacterial chemosynthesis is the major source of food
in all of these communities. No sunlight is necessary and no photosynthetic
production is needed from the surface; the populations of organisms are
sustained entirely by inorganic materials that are converted into bacterial
biomass, which then becomes available for consumption by higher animáis.

8.9.4 UNIQUE ENVIRONMENTAL FEATURES OF SULPHIDE COMMUNITIES

Although hydrothermal vents and coid seeps support communities with high
biomass, species diversity is low compared to that at other deep-sea
localities. Endemic species predominate; over 90% of the animáis found at
vents and seeps do not occur outside their special habitats. The environments
of vents and seeps possess certain attributes that place physiological
constraints on animáis and require special adaptations. Many animal groups
are not represented at these sites, presumably because they have not evolved
the ability to cope with the special conditions. With a few exceptions,
cnidarians other than sea anemones, echinoderms of all types, sponges,
xenophyophores, brachiopods, bryozoans, and fish are uncommon or absent.
Molluscs, polychaetes, and crustaceans account for more than 90% of all
vent species.

Hydrothermal vents, in particular, are transient environments that undergo
large and rapid changes. The geological processes that create vents are
dynamic events, and new vents are being formed as others cióse. Vent
communities probably persist only for several years to several decades. Oíd,
inactive vents are surrounded by the shell remains of clams and mussels that
died when their energy source disappeared.

Animáis living around vents are subjected to high temperature variance and
to an oxygen concentration that can switch rapidly from anoxic to oxic
conditions. There are also short-term fluctuations in H 2 S concentrations.
Salinity in vent plumes varíes from about one-third to twice that of normal

243

seawater. Hydrothermal fluid contains many inorganic substances which
precipítate upon contact with seawater. The chimneys that emit superheated
water are formed from the precipitation of sulphide deposits containing
copper and zinc. This also means that animáis are subjected to a rain of
inorganic precipitates that coat their surfaces, and that they are exposed to
potentially toxic concentrations of dissolved and precipitated heavy metáis.

Both vents and seeps contain very high concentrations of H 2 S (to 19.5 mM
in hydrothermal fluid), and this raises the question of how the animáis
escape being poisoned by the high levels of H 2 S. Hydrogen sulphide, even
at concentrations less than one-thousandth of those found in some vent
animáis, poisons aerobic respiration. Only certain bacteria have the
appropriate enzyme systems to oxidize this molecule in order to obtain
Chemical energy. Preliminary studies on Riftia indícate that it has special
biochemical adaptations that protect the worm from H 2 S toxicity. These
inelude a special sulphide-binding protein in its blood, and enzyme systems
in its body wall that oxidize any free sulphide entering the cells. Other vent
species may be similarly equipped with sulphide-detoxifying systems.

Hydrothermal vent communities are quite small, usually only about 25-60 m
in diameter, and they (as well as coid seeps) may be separated from other
similar communities by as much as hundreds to thousands of kilometres.
Vents are also ephemeral, lasting only on scales of years.

Given these fealures, how can animáis mainlam populations in widespread
local ¡lies and how are new vents populated?

In order to succeed in short-lived, scattered habitats, animáis of these
communities could be expected to grow rapidly to sexual maturity, produce
many young, and have efficient means of dispersal (see Section 1.3,1). Such
r-selected traits would allow them to reproduce within the time span of their
habitat and to continually colonize new vent areas. Although reproductive
studies of vent fauna are few, zoogeographic studies suggest that vent
species do rely on larval dispersal. On-going research into types of larval
development and dispersal mechanisms is seeking information on these
aspeets.

8.10 SUMMARY OF CHAPTER 8

1 Relative to most other marine habitats, intertidal areas are characterized
by great fluctuations in environmental conditions. Littoral plants and animáis
are specially adapted to cope with variable temperatures and salinity, and to
withstand periodic exposure to air.

2 Rocky intertidal regions support dense communities with a high
proportion of epiflora and epifauna that may compete for limited space.

Many of the sessile species are arranged in distinct vertical zones. The upper
boundary of any particular zone is often set by physiological limits of the
species, such as tolerance to desiccation and temperature change. The lower
limits of zones are generally established by biological factors such as
predation and competition.

3 Intertidal areas of sand beaches support communities in which the
primary producers are benthic species of diatoms, dinoflagellates, and
blue-green bacteria, and the resident animáis are predominantly infauna and

244

meiofauna. The meiofauna are specially adapted to live on sand grains, or in
the interstitial spaces between sand partióles, by their small sizes, elongate
shapes, protected integuments, and adhesive organs.

4 Annual primary production averages about 100 g C m -2 in rocky
intertidal areas, with a máximum of 1000 g C m -2 in particularly favourable
areas. Benthic primary productivity in sand beaches is less than

15 g C m -2 yr -1 , and this system relies on energy derived from detritus and
from primary production in the surrounding water.

5 Subtidal kelp forests occur on rocky substrates in coid températe regions.
Kelp are among the fastest growing of any plants, and the productivity of
kelp beds ranges from about 600 to more than 3000 g C m -2 yr -1 . Much of
this production is not consumed directly, but enters the detritus food chain.
Sea urchins are dominant components of kelp communities, and their
feeding activities greatly influence the community structure. In some North
Pacific kelp beds, otters are the top predators that act as keystone species.

6 The circulation pattern of estuaries results in entrainment of nutrients and
makes them some of the most productive of marine ecosystems. The upper
reaches of estuaries are occupied by saltmarsh communities with total annual
primary production generally ranging between 300 and >3000 g C m -2 .
Seagrass beds typically form in intertidal areas of the middle reaches of
estuaries, and total primary production by the seagrasses and associated
epiphytes is about 600 to 1000 g C m -2 yr -1 . Both communities are
dominated by detritus-based food chains. Estuaries also have subtidal
mudflats or subtidal sand banks in which annual primary production (mostly
by epipsammic algae) ranges from 10 to >200 g C m -2 .

7 The high productivity of estuaries supports dense populations of animáis
in some areas. However, many animáis are excluded from living in estuaries
because of the fluctuating salinity, and thus species diversity is low.

8 Coral reefs are formed by stony coráis that contain symbiotic
dinoflagellates called zooxanthellae. The algae utilize carbón dioxide and
waste products of the coral in photosynthesis, and in return the coral is
provided with organic compounds such as glucose and glycerol.
Photosynthetic fixation by the zooxanthellae provides only part of the energy
required by coráis; the remainder is supplied by predation on zooplankton
and bacteria, and by absorption of dissolved organic matter.

9 Primary producers of coral reefs inelude phytoplankton, benthic algae,
and zooxanthellae. Gross primary productivity is very high, ranging from
about 1500 to 5000 g C m -2 yr -1 , but production to respiration ratios
usually are between 1.0 and 2.5, and very little new nutrient material enters
the system (i.e. /-ratio is <0.1). The high production of this system supports
a community with very high species diversity.

10 Coráis grow relatively slowly, at rates of <1 to 10 cm yr -1 . Growth of
a reef is also controlled by bioerosion and physical events (e.g. storms) that
destroy the carbonate framework. Net vertical upward growth of reefs varies
from a few to almost 30 mm yr -1 under favourable conditions.

11 Mangrove swamps occur along 60-75% of tropical and subtropical
coasts. The major primary producers of these communities are salt-tolerant
terrestrial plants that can live in oxygen-poor muddy substrates. The roots of
the mangroves provide attachment sites for epifauna, and leaf fall is a major

245

source of nutrients and energy for the detritus-based food chain. Net primary
production is estimated to be between 350 and 500 g C m“ 2 yr _1 .

12 The bathyal and abyssal zones together constitute over 90% of the
benthic environment. Deposit-feeding infauna predomínate in
organically-rich sediments, and the meiobenthos are particularly diverse.
Benthic biomass diminishes rapidly with increasing depth, and annual
secondary production is between 0.005 and 0.05 g C m~ 2 . Most areas of the
deep sea depend upon the fall-out from production in the euphotic zone, but
only a small proportion of sinking organic matter reaches the seafloor in
depths over 2000 m, and food limitation greatly influences biological
processes and community structure of the deep-sea benthos. In general,
typical deep-sea inhabitants exhibit slow metabolic rates, slow growth rates,
and low fecundity.

13 Ocean trenches characteristically have a high proportion of endemic
species. The biomass of hadal animáis ranges from about 0.008 g m -2 in
trenches far from land and underlying oligotrophic water, to as much as
9 g m -2 in trenches that lie near land under eutrophic water.

14 Hydrothermal vents and coid seeps support unique communities that are
independent of solar energy and photosynthesis. Instead, the food chain in
these environments is based on the presence of hydrogen sulphide that is
utilized by chemosynthetic bacteria to form organic compounds from carbón
dioxide. The bacteria are the primary producers in these communities, and
they are either consumed directly by animáis or they are found in symbiotic
relationships with animáis.

15 Deep-sea vents and seeps support extremely dense concentrations of
large animáis, and biomass may be as much as 30 kg m~ 2 . Although these
environments have plentiful food and, in the case of vents, temperatures that
are higher than usual in deep water, relatively few animáis have developed
the ability to live in high concentrations of H 2 S and species diversity is low.

Now try the following questions to consolídate your understanding of this
Chapter.

QUESTiDN 8.16 Apart from iheir high primary productivity, what important
biological propeity do marshgrass and seagrass communities have in
common that benefits animáis?

QUESTION 8.17 If the re is intense compe t ilion for spaee in the coral reef
ecosystem, can you think of any reason(s) why the fas test growing species
dorTt overgrow or crowd oul other species and thus reduce the species
diversity of the system?

QUESTION 8.18 If sea leve] rose by 2 m in the next 100 years beca use of
el i mate warming* would coral reef growth be able to keep up with the rise of
Coastal waters?

QUESTION 8.19 Although they are characterislic of differem latitudes, tropical
mangrove swamps and températe saítmarshes share certain ecológica]
fealures. What are these common characteristics?

QUESTION 8.20 In lerms of quaniity, how important might crustacean moults
he in ihe sinking of organic materials into the deep sea? Refer back to
Seclion 4.2 for help with your answer.

QUESTION fi.21 What are some of the biológica) or ecológica) advantages
conferred on organismo ihal live at great depths?

QUESIIQN 8.22 Of the two tvpes of marine food chains deseribed in the Jeep
sea, one hased on bacterial ehemosymhesis and one on algal photosynthesis,
vv h i c h wdu íd be evo] u ti o n ari 1 y o 1 d e r? i R e fe r to t he G eo I ogi c Ti i ne Seal e i n
Appendix L)

CUESTION 8.23 (a) Which henthic cotn muñí lies are characterized by having

many endemic spedes? (b) What reason(s) explain the high degree of
endemicity ín these comtnunities?

CHAPTER 9

HUMAN IMPACTS ON MARINE BIOTA

Humans change marine environments and affect marine organisms in many
ways. We harvest marine plañís and animáis, carry out mariculture, reclaim
land, dam rivers that run to the sea, and dredge harbours. Marine organisms
are transported around the world, and are deliberately or accidentally
introduced into new areas. The sea has long been regarded as a convenient
dumping site, and various pollutants are released into the marine
environment from domestic or industrial outfalls or from accidental spills.
Coastal ecosystems become enriched from nutrients contained in sewage, in
discharged detergents, and in agricultural runoff. All of these activities, and
others, may change the species composition of marine communities, result in
loss of marine organisms or loss of marine habitats, or disrupt whole marine
ecosystems. A summary of some examples of human impacts is given in
Tables 9.1 and 9.2.

Table 9.1 Major impacts of industrial activities on marine environments.

Activity

Impact location

Effects

Harvesting lisli

World- wide

Changes in the species
composition of peí agio and
benthic communities

Changes in size structure of
largeted fish populations

Fishing methods

World-wide (dependí ng
apon specific types of
fisheries)

Benthic trawling destroys
bottom habitat

Dynamite fishing destroys
coráis

Unselective fishing meneases
discarded by-catch

Discard of by-catch

World-wide

Increase in scavenging species

Accel eral ion in delivery of
nutrients u> deeper water

Possi ble mercases in benthic
biomass

Dam construcción

Rivers running lo sea

Loss of habitat for anadromous
tish

Urban developtnent

Estuaríes; coral reefs;
mangroves

Land reclamation leads to loss
of habitat

Sewage disposal and
agricultural runoff may cause
eutrophication

Industrial runoff may poIlute
Coastal waters

Com mere i a 1 sh i pp í n g

World-wide

Introducción of species into
new environments

Coral mining

Tropical reefs

Destrucción of coráis

248

9.1 FISHEMES IMPAGTS

Whai human activity has caused ihe greaiest chances in ihc ocean?

The greatest and most serious human impact on marine ecosystems is caused
by the annual removal of more than 100 million tonnes of fish and shellfish
(reported catch plus by-catch, see Section 6.7.1). This harvest affects the
species composition of pelagic communities as well as nutrient
concentrations in surface waters (see discussion of /-ratio, Section 5.5.1).
Mid-water and benthic communities may also be impacted by the dumping
of dead by-catch which delivers a rich source of nutrients to deeper waters.
There are also disruptive habitat changes caused by bottom trawling.

Advances in fishing technology have made it easier to lócate fish schools
and to catch more fish more effectively. At the same time, the world fishing
fleet has increased rapidly, doubling to about 1.2 million vessels between
1970 and 1990. Long lines with thousands of baited hooks may extend more
than 125 km from a ship, and some mid-water trawl nets with a mouth gape
of 130 m and length of 1 km are large enough to encompass the Statue of
Liberty or to extend around 12 jumbo jetliners. In some cases, more than
80% of a commercially lucrative stock is removed each year. These facts are
reflected in Figure 9.1 which shows the increase in marine fish catch from
less than 20 x 10 6 tonnes in the late 1940s to about 85 x 10 6 tonnes in 1993;

Figure 9.1 The marine fish catch (excluding by-catch) from 1947 to 1993, based on FA0
statistics.

249

these figures do not inelude by-catch. However, it has become increasingly
clear that many fish stocks throughout the world are now dwindling and that
the catch-per-unit-effort (CPUE) of fishing has decreased. In the last two
decades, fish catches have declined in all the major oceans except the Indian
Ocean, where modera fishing fleets only began operating intensively in the
late 1980s. A report issued by the United Nations Food and Agriculture
Organization in 1995 concluded that 70% of the ocean’s fish stocks are
either fully exploited, overfished, or recovering from being overfished.

Although ocean climate change may be responsible for declines and changes
in some fish stocks (see Section 6.7.2), overfishing is clearly responsible for
the declines in many commercially favoured species. The 200-year-old
fishery for cod and haddock stocks off eastera Cañada and New England
essentially ceased in the 1990s, although it may recover in time. Figure 9.2
shows how declines in these preferred species have resulted in changes in
the abundance of other fish; skate and dogfish populations have increased by
35-40% since 1965, while cod, haddock, and hake have decreased by 45%.
At the same time, less predation by cod and haddock on young lobsters has
increased this lucrative shellfish harvest. Similar changes in community
species composition have been documented in the North Sea, where
sandlance greatly increased and became a target fish following declines in
herring and mackerel stocks. In the Antarctic, commercial whaling and
dramatic declines in the numbers of whales resulted in increased numbers of

Figure 9.2 Changes in the relative abundance of different fish species on Georges Bank,
U.S.A., between 1963 and 1992, following overfishing of cod, haddock, and hake stocks.
(Data from the National Marine Fisheries Service, U.S.A.)

cod, haddock
and hake

flounder

skate

dogfish

n i ®63

EÜ3 1992

other animáis that were dependent upon krill for food (see Table 5.2). In the
1960s, commercial fisheries in the Black Sea targeted 26 species of fish,
many of them large predators with long life cycles. Overfishing, construction
of dams, and pollution have reduced this to only five commercially viable
species, all of small size. On the other hand, the total biomass of the Black
Sea fishery harvest has actually increased due to the greater abundance of
smaller fish species whose populations are no longer kept in check by
predation, and to increased fishing effort. Overfishing may also cause
changes in size structure of fish populations; for example, the total weight of
spawning Atlantic swordfish fell by 40% between 1978 and 1989. There are
also examples of declines in shellfish populations due to a combination of
overexploitation and Coastal pollution. Within 30 years, oyster catches in
Chesapeake Bay, in the eastern U.S., fell from 20000 to 3000 tonnes.

Anadromous fish, such as salmón, are adversely affected by the dams that
block access to spawning areas. Despite expensive efforts to enhance stocks
through hatchery rearing and construction of runways around dams, salmón
stocks in the eastern North Pacific Ocean have declined in some rivers (e.g.
the Columbia River). Loss of Coastal spawning grounds and nursery areas
through land development and/or pollution is another increasing problem for
many fish species. Fishing activities may also destroy habitats. Heavy fish
trawls can penétrate 6 cm or more into the seabed, thereby disrupting the
natural substrate and releasing nutrients into the water column, and
destroying zoobenthos that may be food for the demersal fish stocks. In
particularly rich fishing grounds, more than 70% of the sediment can be
ploughed by trawls.

Commercial fisheries discard about one of every four animáis caught; the
percentage of this unwanted by-catch may in fact be larger because much
by-catch goes unreported. The discarded catch ineludes species with no
economic valué and young fish that are too small to market. In some cases,
the by-catch may exceed the target catch. Shrimp fisheries have an
exceptionally large by-catch. The shrimp fishery in the Gulf of México
catches and discards at least 5 million juvenile red snapper annually, or
4.2 kg of fish for every 1 kg of shrimp. World-wide, the total by-catch of
shrimp fisheries may be as much as 17 million tonnes per year; much of this
is made up of small fin-fishes, most of which do not survive capture and
release. Discarded by-catch increases the number of scavengers in fishing
grounds. In the North Sea, it is estimated that the annual discarded by-catch
of about 90000 tonnes of whitefish can potentially support about
600000 gulls.

Unfortunately, as a valuable fishery species becomes more scarce, its
economic valué tends to increase. Thus it often remains profitable for fleets
to continué to take an overfished species. Spawning populations of bluefin
tuna (the world’s most valuable fish) have declined by about 80% in the
western Atlantic since 1970, and by 90% in the Gulf of México since 1975,
but a single large specimen may fetch more than US $80 000 (about
$265 per kg in 1996) on the market.

Declining world fish catches have alerted nations to the fact that it is indeed
possible for man to deplete fish populations and to alter oceanic ecosystems
over vast regions. Some progress has been made to alleviate the problems of
overfishing; regulations have been set concerning allowable sizes and total
catch for some species, driftnets have been largely banned, tuna fishers have
adopted new methods to avoid capturing and killing dolphins, and whales

251

are no longer the targets of commercial fishing. But International regulations
are difficult to establish and to enforce on the open seas, and economic
issues, not scientific management, continué to drive the industry. Hopefully
the next decade will see the resolution of global fisheries problems before
entire fish stocks and pelagic ecosystems are irrevocably changed.

9.2 MARINE POLLUTANTS

Marine pollution has been defined by the Intergovemmental Oceanographic
Commission as the introduction by humans, directly or indirectly, of
substances or energy sources into the marine environment resulting in
deleterious effects such as harm to living resources; hazards to human
health; hindrance to marine activities, including fishing; impairment of the
quality of seawater; and reduction of amenities. The number of different
pollutants entering the sea is very large, and new substances are added every
day. Some of the substances regarded as pollutants, like heavy metáis and
Petroleum hydrocarbons, occur naturally in the sea and human introductions
add to natural concentrations. ¡Some introduced pollutants will decompose in
time or will be attenuated by the very large volume of the oceans, so that
their effect will not be noticeable. Other pollutants may have significant
impacts. Some of the major anthropogenic pollutants and their effects are
summarized in Table 9.2 and discussed.below.

Table 9.2 Some major forms of marine pollution and their effects.

Poli mam

Local ion

Effect

Peiroleum

hydrocarbons

Local otl spills

Mass mortality of benlhos and
seabirds

World-wide seas

Lo w- l e ve 1 concentrati on efTects
unknown

Plastics

Bcaches

Aeslhetically dísturbing

Floating debris

Entanglement of animáis;
ingestión by animáis

Pesticides and
related compounds

Local point-source inputs

Acute toxicity

World-wíde seas

Long-term sublethal effects
largely unknown

Heavy metáis

Industrial outfalls

Mostly sublethal effects
catising growth abnormalities

Se w age

Local outfalls;
agri cultural runoff

Eutrophication and allcrcd
community strocture;
introduction of pathogens

Radioactivo w as tes

Local power plants;
historical at-sea dumping
siles

General ly considered to be
helow ha mi ful le veis

Thermal effluents

Local power plants

Warming leads to a he red
co m m u n i ty si ruct u re

252

9.2.1 PETROLEUM HYDROCARBONS

Petroleum hydrocarbons have probably attracted the most attention as marine
pollutants because the impact of an oil spill is visually very apparent.

Table 9.3 lists some of the major spills, the largest having occurred during
the Arabian Gulf war when approximately one million tonnes of oil were
spilled into the Gulf of Arabia. The largest spill from an oil tanker occurred
when the Amoco Cádiz went aground off Brittany in 1978, releasing 220000
tonnes of crude oil (Colour Píate 41). More than 300 km of shoreline were
affected, causing the elimination of at least 30% of the marine benthic fauna
and the death of some 20000 birds. The effects of the Exxon Valdez spill
(ca. 30 000 tonnes of oil) on Alaskan populations of birds and otters were
noted in Sections 6.5 and 8.3. Although such large spills are devastating
within localized areas, the natural recovery time of shoreline communities,
under modérate conditions of wave action, is usually within 5 to 10 years for
most organisms, although bird and otter populations may take longer to
recover because of their slower reproductive rates.

Table 9.3 Some major oil spills in the ocean. (Italicized ñames are of oil tankers.)

Locatíon

Source

Amuunt of
oil spilled
(tonnes)

Dale

Arabian Gulf

Gulf War

1 000000

1990-91

Gulf of México

Oil well

440 (XX)

1981

Brittany. France

Amoco Cádiz

220000

1978

Comwall, U.K,

Torrey Canyon

117000

1967

Wales. U.K.

Sea Empress

70 (XX)

1996

Japan Inland Sea

Storage tank

8-40000

1974

Alaska, U.S

Exxon Valdez

37 000

1989

Northwest Atlantic

Argo Merchartt

30000

1976

North Sea

Oil well

15 (XX)

1979

Tanker accidents are responsible for only a small percentage of the oil
entering the sea. The production and transportation of oil, conventional
shipping, waste disposal, and runoff are all additional sources of oil in the
marine environment. There are also natural seeps, where oil deposits cióse to
the Earth’s surface leak into the sea. It is estimated that between 2.5 and 5
million tonnes of petroleum hydrocarbons enter the ocean each year from all
sources. Over time, very small amounts (ppb) of petroleum hydrocarbons
have accumulated world-wide in the oceans. The experimental toxic effect of
such hydrocarbons is generally at concentrations of parts per million (ppm),
and therefore the present accumulated background is not considered harmful
to marine organisms, although effects of long-term chronic exposure to such
concentrations are not fully known.

QUESTIDN 9.1 In wbat type of marine environment would you ex pee t to tind
the ¡dowest rale of recovery from a. large oil spill?

9.2.2 PLASTICS

Discarded plástic materials in the oceans range in size from large nylon drift
nets (see Section 6.2) to pellets of less than a millimetre in diameter which
can be distributed by the wind over the whole ocean. These materials are not
biodegradable; although plastics do break down as a result of physical and

253

Chemical weathering, this process is slow and therefore plastics accumulate
over time in the sea. Loose driftnets (or ‘ghost nets’) or other discarded
fishing gear, for example, can continué to entangle marine animáis for years
before washing ashore or sinking. Plástic bags or small plástic pellets are
often mistaken for prey and ingested by marine turtles and seabirds,
respectively. Pellets have been found in at least 50 species of marine birds.
A survey of shearwaters (Pujfinus sp.) in the North Pacific Ocean revealed
that more than 80% of 450 birds had plástic partióles in their stomachs.
Ingested plástic bags are known to kill turtles; although one can speculate
that ingested pellets may be harmful to birds or other marine life, there is
presently no direct evidence to support this.

QUESTIQi 9.2 Can yon think of any ways lo reduce the amoum of plástic
material that entera the sea eaeh year?

9.2.3 PESTICIDES AND OTHER BI0L0GICALLY ACTIVE ORGANIC C0MP0UNDS

The most common pesticides entering the oceans are various forms of
chlorinated hydrocarbons. These man-made compounds do not occur
naturally, they are not readily degraded by Chemical oxidation or by bacterial
action, and they accumulate in animal fat tissues because they are
lipid-soluble.

The best-known insecticide, DDT, was first employed in 1940, and within 20
years it and its residues could be found throughout the biosphere. Because
DDT was often sprayed from aircraft, it was easily carried by winds into the
oceans. Eventually even Antarctic penguins, living several thousand
kilometres from any place where DDT had been used, were found to contain
ppb traces of DDT. During the 1960s, there was increasing evidence that
marine organisms, particularly seabirds, were being adversely affected in
marine areas where DDT concentrations were exceptionally high. One
example occurred off Southern California, where a pesticide company had
released DDT for 20 years into the coastal environment. DDT entered the
ocean food chain, and its effects on the marine biota could be detected for
100 km along the coast. Fish in this area contained >3 ppm DDT, and
pelicans and sea lions that fed on the fish accumulated even higher tissue
concentrations of DDT and were unable to breed successfully. Even
following a dramatic reduction in DDT emissions after 1971, DDT levels in
fish remained high for years. Harmful effects were found in the Los Angeles
Zoo in 1976, with the death of all the cormorants and gulls that had been fed
inadvertently for several years on locally caught fish contaminated with
DDT. Autopsy results found DDT concentrations ranging between 750 and
3100 ppm in liver tissues of these birds.

DDT usage has now been banned in some countries, but continúes to be
used in tropical areas as an effective control against mosquito populations
that carry malaria. At present, most of the chlorinated hydrocarbon in the
sea, and 80% of that in marine organisms, is in the form of DDE, a Chemical
derived from the breakdown of DDT. In most surface waters, DDT/DDE
concentrations are between 0.1 and 1 ng l -1 (or less than one part per
trillion), and these levels are not considered to be harmful. It is now
recognized that heavy use of synthetic pesticides in agriculture is associated
with significant undesirable side effects, and efforts are being made to find
alternatives through biological or genetic Controls of pests.

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In addition to pesdcides, several toxic chlorinated hydrocarbons are used
industrially and may be present in seawater. These inelude dioxins and PCBs
(polychlorinated biphenyls), both of which may have deleterious effeets on
marine life. PCBs are stable compounds that tend to persist in the
environment and to be concentrated through biological processes. These
characteristics were underscored by the discovery of exceptionally high PCB
concentrations in several beluga whales that died in the St. Lawrence River
in 1985; they contained up to 575 ppm in lipid tissue and 1750 ppm in the
milk. The recognition of environmental problems has resulted in a ban on
production and usage of PCBs in the United States. In the mid-1960s, the
organo-tin compound tributyl tin (TBT) was found to have exceptional
antifouling properties, and was consequently applied to boat hulls and
fishing nets to prevent the settlement and growth of marine organisms.
Unfortunately it leaches into surrounding waters, where concentrations
between 0.1 and 100 /xg l -1 are toxic to the larvae of many benthic
invertebrates, and levels as low as 0.001 /xg l -1 may affect reproduction in
some marine snails. Ship traffic has spread TBT globally and the compound
has accumulated in sediments near harbours and ports. Several countries
have now established regulations designed to curb TBT usage.

9.2.4 HEAVY METALS

Heavy metáis such as mercury, copper, and cadmium occur naturally in
seawater at low concentrations, and they enter the sea through natural
erosión of ore-bearing rocks and subsequent transpon in rivers or via dust
particles in the atmosphere, and through volcanic activity. All of these metáis
can be poisonous to organisms in high concentrations, and thus potential
health problems exist where heavy metáis accumulate in the sea around
industrial outfalls, or at marine sites used to dispose of some types of mine
tailings. At such localities, the local benthos may accumulate metáis in levels
exceeding permitted concentrations for marketable marine producís (the
permitted level for mercury is 1 ppm). Generally, heavy metáis have acute
toxic effeets, but accumulations of these substances in marine animáis may
also cause chronic effeets such as growth abnormalities, including cancers.

A serious case of heavy metal poisoning in humans occurred in Minamata,
Japan, where a plastics factory discharged an estimated 200-600 tonnes of
mercury over a period of 36 years into the local bay. Illnesses began to
appear in the early 1950s, and by 1956 these were diagnosed as mercury
poisoning derived from eating contaminated shellfish and fish from the bay.
Effeets included severe neurological damage, paralysis, and birth
deformities. By 1988, 2209 victims had been verified, of whom 730 died.
Following this tragic discovery of the dangers to human health from eating
mercury-containing seafood, regulations were adopted in many countries to
limit mercury discharges and to limit the tissue concentrations allowed in
seafood producís.

What are normal concentrations of mercury in the muscle tissue of pelagic
fish?

Most species of fish in uncontaminated oceanic waters contain about
150 /xg kg -1 (0.15 ppm) of mercury in their muscles. However, some large
pelagic species such as sharks, swordfish, black marlin, and tuna may have
tissue concentrations as high as 1-5 ppm, but these levels are not indicative
of anthropogenic pollution. These long-lived fish are large carnivores at the

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end of food chains, and their high mercury levels are acquired by
bioaccumulation over their life spans.

Although other heavy metáis, such as cadmium, copper, and lead, may
accumulate in marine organisms exposed to high concentrations at waste
discharge sites, they are not known to have caused serious human health
issues. Problems arising from the use of another metal, tin, have been
discussed in Section 9.2.3. International agreements now regúlate marine
discharge and usage of some of the more dangerous metáis.

9.2.5 SEWAGE

Sewage disposal is a major form of Coastal pollution throughout the world.
Sewage outfalls near Coastal communities release human waste as well as
other organic matter, heavy metáis, pesticides, detergents, and petroleum
producís. Nutrients from organic waste material may cause eutrophication;
local waters may also be nutrient-enriched by detergents that contain
phosphate and by agricultural and horticultural producís entering from
runoff. In addition, human sewage delivers pathogenic bacteria and viruses
that are not necessarily killed by exposure to seawater; high concentrations
of these microbes make local seafood unsafe to eat and contaminated waters
unsafe for bathing. The chief health risk from sewage is through eating
contaminated seafood, particularly filter-feeding clams or mussels which
accumulate human pathogens on their gills. The cholera virus is a particular
problem in some countries, and may be transmitted in just such a manner.

In urban areas of developed countries, sewage may receive special treatment
to degrade organic matter or to remove nitrates and phosphates, but these
processes are expensive. Usually no more sewage is treated than is deemed
necessary, and in many places sewage is released into the sea without
treatment. Generally the immediate area (within 100 m) around a large
sewage outfall may be anoxic and dominated by anaerobic bacteria. At some
greater distance from the outfall (within several km), nutrient enrichment
typically leads to increased production of green macroalgae (Enteromorpha
or JJlvá) that form thick mats along the shoreline. A few opportunistic
animáis, such as the polychaete Capitella , are also indicative of sewage
enrichment and may dominate affected benthic communities. At some ten
kilometres from a major domestic outfall, there is usually sufficient
attenuation of pollutants that community species diversity is not affected.

9.2.6 RADIOACTIVE WASTES

Radioactive wastes enter seawater from nuclear testing, from nuclear power
plants or reprocessing reactors, or from delibérate dumping of waste
materials. Heavy radionuclides have low solubility in water and tend to be
adsorbed onto particulate matter; they therefore accumulate in sediments.
Isotopes with long half-lives (e.g. caesium-137, strontium-90, and
plutonium-239) are especially hazardous and are usually monitored in areas
where they may escape from nuclear facilities. Barring major accidents,
background levels in the marine environment around radioactive outfalls are
generally regarded as safe. The potential for reaching high concentrations of
radioactive materials exists in certain localized areas, notably around the
several known sunken nuclear submarines, from nuclear dump sites at sea
(which are now prohibited), and from nuclear testing that has been carried
out within coral atolls (most recently in the South Pacific by France).
However, it is predicted that leakage from such sources would occur at a

slow rate and that there would be dilution of soluble radionuclides and
adsorption of others on to bottom sediments.

Some marine organisms (e.g. seaweeds and bivalves) may accumulate
radionuclides from surrounding water. For example, the alga Porphyra
umbilicalis , growing in the vicinity of a reprocessing plant in England,
accumulated 10 times the concentration of caesium-137 found in the ambient
water and 1500 times the concentration of ruthenium-106. The experimental
consequences of low-level doses of radiation on marine organisms are the
same as those for terrestrial species and may inelude increased incidences of
cancers, impaired immune systems, and genetic defeets causing growth
deformities. However, present levels of radiation in the sea have not
produced any measurable environmental impact on marine biota.

9.2.7 THERMAL EFFLUENTS

Power plants may discharge several hundred thousand cubic metres of
cooling water per hour into Coastal waters, and this thermal effluent may
raise the local seawater temperature by 1-5 C°. In some areas, this warmed
water can be used beneficially to enhance growth rates of organisms grown
in mariculture. However, in mañy cases elevated temperatures cause
unwanted changes in the natural fauna and flora. For example, a persistent
elevation of the ambient temperature by 5 C° along the subtropical Florida
coast resulted in the replacement of natural algae and seagrasses by mats of
cyanobacteria. In another example, the increase in water temperature of a
températe area of the U.S. allowed the entry of warm water wood-boring
bivalves which caused damage to boats and wharves. In most cases, the area
affected is limited to the plume of hot water and its immediate surroundings,
an area that may range from less than one hectare to about 40 hectares.

There may be other effeets from thermal effluents. They often contain
chlorine, which is added to intake water to prevent fouling organisms from
blocking pipes, and as little as 0.1 ppm of chlorine remaining in the effluent
can be toxic to some organisms. Water flow of the plume also mechanically
scours the seabed and so influences the fauna.

QUEST10N 9.3 Ai what tíme of the year would yuu expeci to have the
greatesl impact from thermal effluents released from power plañís Incaled on
a subtropical coast?

9.3 INTR0DUCTI0NS AND TRANSFERS 0F MARINE
ORGANISMS

The movement of species from one región to another occurs naturally
through larval drift, rafting, and other means, but humans have accelerated
these movements and eliminated natural ocean barriers by accidental or
delibérate introductions of species into new areas. In many cases, introduced
species fail to develop reproducing populations, but in some instances an
exotic species encounters favourable conditions and causes a significant
impact on its new environment.

Marine organisms have often been introduced deliberately into new
environments for mariculture purposes. For example, the Japanese oyster
(Crassostrea gigas ) and the east coast oyster (C. virginica ) were brought to

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the north-west coast of North America because they were larger and grew
more rapidly than the local oyster ( Ostrea lurida)\ simultaneously, predatory
snails that bore into oysters and other molluscs were accidentally introduced
and both the oysters and this predator are now firmly established in
this región.

Increased demand from the mariculture and aquaria industries and increased
ship traffic have accelerated the rate at which species are transported to new
environments. Each day, approximately 3000 species of marine animáis and
plants are being carried across oceans in the ballast tanks of ships that take
on seawater in port and then release it at later ports of cali. Examination of
ballast water released in Oregon from a Japanese vessel contained 367 taxa
of both holoplankton and meroplankton. Thus entire planktonic communities,
including the larvae of benthic organisms, can be transported across natural
oceanic barriers.

The release of the ctenophore Mnemiopsis leidyi (see Section 4.7) into the
Black Sea is one example of an invasive species that encountered favourable
conditions and no natural predators, and rapidly became so abundant in the
Azov and Black Sea that it caused drastic declines in the resident
zooplankton and subsequent declines in the anchovy fisheries of bordering
countries. More recently, two species of jellyfish that are native to the Black
Sea have appeared in San Francisco Bay (Figure 9.3); it is too early to
assess their impaets on the ecosystem. Asian species of copepods are now
present in Californian harbours, and the Chinese clam Potamocorbula
amurensis has become one of the most abundant marine animáis in the
estuary of San Francisco Bay. The zebra mussel Dreissena polymorpha ,
originally from the Mediterranean, entered the North American Great Lakes
from ballast water in 1986 and almost immediately began to grow in such

Figure 9.3 Maeotias inexspectata, a jellyfish native to the Black Sea that has recently been
introduced into San Francisco Bay, California.

profusión that it blocked water intakes, resulting in an expense of ten billion
dollars to deal with its destructive effects.

Australia has had over 35 successful invasions in recent years; the predatory
starfish Asterias amurensis has invaded the coast of south-eastern Australia,
and a common Japanese kelp, Undaria pinnatifida , is spreading along parts
of Australia at a rate of 55 km a year. It is believed that the appearance of
red tides (see Section 3.1.2) for the first time in Tasmanian waters resulted
from toxic dinoflagellates being transponed in ballast water of cargo ships;
one such vessel contained more than 300 million toxic dinoflagellate cysts in
the sediment accumulated in its tanks.

How could the spread of marine organisms by transpon in hall asi water be
red ti c e d ?

Ships take on ballast water in Coastal seaports. The problem of global
transport of marine species could be ameliorated by requiring ships to
exchange ballast water at sea. Coastal organisms are unlikely to survive in
oceanic water, and oceanic organisms are much less likely to survive release
into low salinity Coastal water.

9.4 IMPACTS ON SPECIFIC MARINE ENVIRONMENTS

9.4.1 ESTUARIES

Many of the world’s largest cities are located on estuaries; London,
Shanghai, and New York are but a few examples. It is not surprising,
therefore, that estuaries have suffered more from human impact than most
other marine environments (see Figure 8.6). The most severely affected area
is usually the saltmarsh community because this upper intertidal area is
easily reclaimed for housing and industrial activity and, sometimes, for
airport construction. In some localities, more than 90% of this community
has disappeared. The deeper seagrass and mudflat communities are often
disrupted by dredging operations to make large harbours and deepen
shipping channels.

The same features that make estuaries productive also make them especially
vulnerable to pollution. Just as nutrients are retained within the System (see
Section 8.5), so are pollutants like petroleum byproducts, heavy metáis,
fertilizers, and pesticides. Both the plankton and benthic communities are
affected by domestic and industrial outfalls releasing organic pollutants that
may cause unwanted eutrophication. Pathogenic organisms, heavy metáis,
and pesticides that enter estuaries may all eventually work their way up the
food chain into edible producís for humans.

Because estuaries are naturally productive, they are often favoured fishing
grounds and are frequently used for harvesting shellfish or for developing
mariculture. Recently, however, many estuarine fisheries and shellfish beds
have been closed due to high numbers of coliform bacteria from domestic
sewage, or from the accumulation of pesticides or heavy metáis in fish
producís. All of these forms of pollution may also alter the structure of the
estuarine ecosystem so that traditional spawning grounds and nursery areas
for fish may be lost. In extreme cases of pollution and eutrophication, anoxic
zones may occur where only bacteria can survive.

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Because of the dangers of human health risks from pollution of estuarine
waters, monitoring programmes have been established in developed
countries. Many pollutants occur in very low concentrations (ppb level) in
seawater and are therefore extremely difficult to measure, even using
sophisticated equipment and techniques. For this reason, a special programme
called Mussel Watch was established to monitor concentrations of marine
pollutants in mussels. This programme uses mussels ( Mytilus) because they
are abundant world-wide in Coastal regions, because they are sessile animáis
that are exposed to any pollutants contained in the water flowing over them,
and because these filter-feeding bivalves are known to accumulate a variety
of pollutants. They have also been extensively studied, both experimentally
and ecologically. Tissue levels of pollutants that have accumulated in
mussels from contaminated areas can be more easily measured than pollutant
concentrations in ambient waters, and these valúes can be compared with
standards in mussels from uncontaminated regions. Since 1986 in the United
States, mussels (or oysters) have been collected from over 200 localities
once each year. Tissues are analysed for several heavy metáis, chlorinated
hydrocarbons including DDT and PCBs, and tributyl tin. Analysing these
pollutants in mussel tissues is much easier and cheaper than analysing the
same substances in trace concentrations in seawater.

QUESTIOfi 9.4 Drawing on your general knowledge of geography, can yon
think of any large estuarios in the world that do noi have adjacem cutes and
are not affected by human populations?

9.4.2 MANGROVE SWAMPS

Mangrove swamps (see Section 8.7) suffer from many of the same
environmental disturbances that are experienced by estuaries. Dredging, land
reclamation, garbage and sewage dumping are all disturbances that can have
significant impacts on mangroves near populated areas. In these tropical and
subtropical ecosystems, insect control (particularly of malaria-carrying
mosquitoes) has resulted in accumulations of pesticides in estuarine
sediments and in mangrove food chains. During the Vietnam war, spraying
of herbicides on mangrove swamps defoliated and destroyed as much as
100000 hectares. Oil spills smother both algae and invertebrates, and disrupt
the oxygen supply to the root system. Where river water has been diverted
into irrigation systems, the reduction in freshwater discharge and the
resulting elevated salinities may be detrimental; for example, a considerable
area of mangrove swamp has been destroyed by diversión of water flow
from the Indus River in Pakistán.

Overcutting of mangroves is, and has been for centuries, a serious problem
in many areas. Mangroves once existed along the shores of the Persian Gulf,
where they were a much-needed source of firewood for humans and of green
fodder for camels in a desert environment, but they were eventually
eliminated by overcutting. Some efforts had been made to re-establish
mangroves along north-eastern Saudi Arabia, but these were destroyed by
the Gulf War. Other countries, recognizing the benefits of mangroves, have
also developed afforestation programs, reintroducing mangroves with varying
degrees of success. Globally, however, destruction of mangroves is
progressing faster than reintroduction. Almost half of the world’s mangroves
have been eliminated in recent years in order to build shrimp farms or rice
paddies. In countries like Bangladesh, removal of this buffering zone has led
to intensified coastline inundation and erosión from tropical storms.

260

Typhoons and hurricanes remain perhaps the greatest destructive agents of
mangrove swamps, as they affect very large areas and occur frequently. Not
only do they uproot trees, but severe storms alter the salinity of both water
and soil, and they cause massive sedimentation. It is estimated that recovery
of mangrove forests from very violent storms takes at least 20 to 25 years.
Whereas little can be done to reduce damage from natural events, it is
possible to develop management policies for the exploitation of mangrove
resources, including replanting. The rational utilization of mangrove areas
depends ultimately on increasing public awareness of the importance of this
unique marine community to local populations in developing tropical
countries.

9.4.3 CORAL REEFS

Coral reefs not only have great beauty and support a very high natural
diversity with many endemic species, but they also have wider biological
and economic importance. Because coráis remove large amounts of bound
CO 2 from the oceans during calcification, the reefs play a role in the global
CO 2 budget. They are of benefit in protecting coastlines and providing
sheltered harbours. And as air travel has become cheaper and more
available, reefs have brought in more and more Tourist dollars’ to boost the
economy of human populations living in their vicinity.

However, coral reefs are extremely vulnerable to disturbance, and reefs are
presently regarded as declining. The World Conservation Union and the
United Nations Environment Program (UNEP) reports damage or destruction
of signiñcant amounts of reefs in 93 out of 109 countries. Much of this
destruction results from human activities, some from changes in ocean
climate.

Expanding human populations near reefs often result in the addition of
various types of pollutants to near-shore waters. These can inelude
agricultural runoff, pesticides, industrial pollutants, and sewage from
beachfront hotels or Coastal communities. Many of these sources increase the
nutrient concentrations in the seawater, and this eutrophication triggers
outbursts of benthic algae that can outeompete coráis for available space.
Often the algae overgrow the coráis, smothering and killing the reefs by
cutting off the sunlight required by the zooxanthellae. Just such an event
followed a burst of land construction and development in an area of the
Hawaiian Islands. After thick mats of algae overgrew and killed large areas
of the reef, the decomposition activities of bacteria led to a lowering of
oxygen concentrations in the seawater. The final result was a dramatic
decrease in diversity, with a particular sea cucumber becoming the
dominant animal.

Various types of Coastal development also result in increased land erosión,
which then increases the amount of sediment in water overlying the reef.

The suspended silt decreases light penetration, thus reducing photosynthesis
of the zooxanthellae and diminishing a nutritional source for the coráis.
Although coral polyps are capable of removing some settling sediment,
using mucus trapping and cilia to cleanse their surfaces, excessive quantities
of silt will clog this apparatus and smother the polyps. Deforestation, leading
to increased runoff and excessive sedimentation, is a major cause of coral
reef destruction. Logging in one area of the Philippines (Bacuit Bay) has
increased erosión and killed 5% of the area’s reefs. Dredging to deepen

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harbours or open ship channels through the reef has similar effects on
adjacent reefs.

A coral reef offers a number of resources used directly by humans. In some
localities, coral is cut and used as a favoured building material (Figure 9.4).
In the Maldive Islands (Indian Ocean), about 200000 m 3 of coral rock have
been mined in the past 20 years; this represents about one-third of the
available coral in shallow water. This practice has also destroyed large areas
of reefs in French Polynesia and Thailand.

Local inhabitants have traditionally relied on fish from nearby reefs as an
abundant protein source, but demands have increased for this resource as
populations have grown in size and as expanding numbers of tourists enter
the communities. At the same time, private and public aquaria have
increased in number throughout the world, and the capture of exotic ñsh for
sale to the aquarium trade has become an extremely lucrative business,
worth about $40 million per year (in 1996). To meet these increased
demands, traditional fishing methods often have been replaced by much
more detrimental techniques. It has become common to blast with dynamite
to stun fish, which are then easily collected when they rise to the surface. At
the same time, portions of the reef are destroyed, and it is estimated that it
may take some 40 years for areas destroyed by blast fishing to recover to
50% live coral cover. Cyanide is also used to stun fish for live collection,
although it may result in the death of the fish and certainly kills other reef
species. In the Philippines alone, an estimated 150 tonnes of sodium cyanide
is used annually for fish capture. Increased fishing, whether by traditional or
non-traditional methods, has resulted in overexploitation of many species in
many regions. Where destructive fishing techniques have been banned,
regulations often are not enforced.

Fish are not the only reef inhabitants that are removed for consumption or
trade. Coráis are removed for ornamental purposes; about 1500 tonnes of
such coral was imported into the United States alone in 1988. Spiny lobsters,

Figure 9.4 Mined coral that has been cut for building purposes in the Maldive Islands,
Indian Ocean.

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sea cucumbers, and sea urchins are among some of the reef animáis that are
considered delicacies in many parts of the world. Snails and clams are
collected for food, or to sell to tourists and shell collectors. Remo val of large
numbers of animáis from reefs may alter the ecology. For example, sea
urchins are responsible for removing part of the reef framework during
grazing; this bioerosion may be intensified when their natural predators
(some fish and molluscs) are overfished.

Tourism may strengthen the economy of reef areas, but it often has the effect
of damaging or destroying that very resource which people pay to see.
Tourism leads to increased development, which may increase erosión and
siltation over the reef, and it almost certainly increases the amount of
sewage entering the water. As more tourists arrive, more fish is sold to
restaurants and hotels, and more shells and coral are removed as mementos.
Even seemingly innocent activities can have repercussions on the reef
ecology. Reefs off Florida in the United States have been seriously damaged
by amateur boaters colliding with submerged coral heads or anchoring to
coráis which are easily broken in the process. Even walking on reefs at low
tide is destructive to coral. Tropical countries that wish to have long-term
economic benefits from their local reefs would do well to avoid
overexploitation, and to edúcate both local populations and visiting tourists.

In the 1970s, attention was drawn to Pacific reefs that were being destroyed
by Acanthaster planci, the crown-of-thorns starfish. This large (30-40 cm
diameter) echinoderm feeds on coral polyps, but is normally present in low
enough numbers that reef damage is slight. However, Acanthaster began to
undergo population explosions on many reefs in the western Pacific,
including the Great Barrier Reef. Tens of thousands of starfish were found
on some reefs, and the impact of so many large predators was devastating,
with entire reefs being destroyed in some areas. For example, in less than
three years, Acanthaster destroyed approximately 90% of the coral along
38 km of reef off Guam. Parts of the Great Barrier Reef have experienced
major damage from predation during two sepárate starfish outbreaks, one in
the 1960s and again in the 1980s.

Many causes have been advanced to explain the outbreaks of Acanthaster .
These inelude increases in waterborne pollutants or increases in
sedimentation due to dredging or other activities. It also was suggested that
shell collecting was to blame. Many tritons had been removed from reefs, as
the shell of this large snail is a prized ornament. Tritons are one of the few
natural predators of the crown-of-thorns starfish, and the decline of these
snails could result in lessened mortality of Acanthaster. It seems, however,
that no one explanation applies to all the reefs damaged by starfish
predation. There have been suggestions that Acanthaster density fluctuations
may be natural eyeles that are linked to ocean climate change. As with the
similar kelp-urchin interactions discussed in Section 8.3, the debate
continúes about whether starfish population explosions are a contemporary
phenomenon linked with human activities, or whether they are an ecological
pattern that has persisted for thousands of years. In any case, restoration of
coral cover takes place within 10 to 20 years, but it may take much longer to
re-establish the original species diversity.

Storms, exposure during exceptionally low tides, and other natural events
may cause widespread coral damage. In 1982-83, a rapid 2-4 C° rise in
seawater temperature to nearly 30°C was caused by a particularly strong El
Niño event, and this damaged or killed 95% of the coral in the Galápagos

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Islands, and 70% to 90% of the coráis in the Gulf of Panama and in
Indonesia. In 1987, reefs throughout the Caribbean were affected by a
similar event. Coráis are particularly sensitive to elevated temperatures as
they live so cióse to their upper temperature tolerance limits. Any sustained
temperature increase usually results in bleaching due to loss of zooxanthellae,
and eventually to death if the thermal stress continúes. In Hawaii, bleaching
of coral has been correlated with discharge of heated effluent from a power
plant, and bleaching can be experimentally induced by elevated temperature.
On a world-wide basis, global warming poses a serious threat to coral reefs.

Reef-building coráis similar to modern species have a geological record
dating back about 250 million years, but other types of coral reefs occurred
as long ago as 500 million years. More than 5000 species of extinct coráis
are known, compared with the present number of less than 600 reef-building
species. The pace of species extinction may be accelerating in many areas.
Some countries, recognizing the benefit of adjacent reefs, have developed
governmental policies to protect them. About 65 countries now have almost
300 protected areas that inelude coral reefs; these inelude marine reserves
and underwater parks. The economic benefits of controlled tourism may push
other countries to develop conservation policies.

9.5 SUMMARY 0F CHAPTER 9

1 The annual fish harvest of more than 100 x 10 6 tonnes has had a greater
impact on the ocean than any other human activity. By 1995, ñsh catches
were declining in all major oceans except the Indian Ocean and 70% of the
ocean’s fish stocks were either being fully exploited, were overfished, or
were recovering from being overfished.

2 The results of intensive fishing inelude: declines in targeted fish stocks
and consequent changes in relative abundance of species; changes in size
structure of fish populations; declines in pelagic and benthic animáis
captured incidentally as by-catch; acceleration of nutrient transfer to deep
water through dumping of dead by-catch; increased numbers of scavengers
in marine food chains receiving large amounts of by-catch; and destruction
of seabed habitats through benthic trawling.

3 Fish stocks are also affected by construction of dams that eliminate
spawning grounds (e.g. salmonids), and by loss of coastal spawning and
nursery grounds due to land reclamation or pollutiott.

4 Human activities result in the release of a variety of poliutants into the
sea. These substances, which may cause deleterious changes, inelude
Petroleum hydrocarbons, plastics, pesticides and related chlorinated
hydrocarbons, metáis, fertilizers, and radioactive wastes. Sewage outfalls
deliver many of these poliutants as well as human wastes, detergents, and
pathogenic bacteria and viruses. Power plants also release heated effluents
that elevate ambient seawater temperatures.

5 Although oil spills are among the most visible types of marine pollution
and ecological damage in the immediate site may be severe, populations
generally recover within 5 to 10 years. In the open ocean, the accumulated
concentration of petroleum hydrocarbons is too low to cause measurable
effeets.

264

6 Nonbiodegradable plástic materials are now found throughout the oceans.
These inelude lost fishing nets, which may continué to entangle animáis for
years, and plástic materials that are mistaken for prey and ingested by turtles
and seabirds.

7 Toxic synthetic pesticides like DDT and related compounds (dioxins,
PCBs) that enter the marine environment are not readily degraded; they
persist for long periods and enter marine food chains. Because they are
stored in fat tissues, these compounds show biomagnification, with higher
trophic level animáis accumulating concentrations that may be lethal. Past
incidences of pesticide and PCB poisoning in marine organisms have led to
bans on usage and production in some countries.

8 Accumulations of heavy metáis (e.g. mercury, copper) resulting from
industrial outfalls may cause serious human health problems. Historically,
the damaging effeets caused by humans eating mercury-contaminated
seafood were shown in the 1950s in Minamata, Japan, where more than
2000 people were directly affected. Now regulations and monitoring
programmes exist to limit and detect unacceptable concentrations of these
metáis in marine produets.

9 Sewage disposal is a major form of Coastal pollution throughout the
world. Nutrients in human wastes and those in detergents and fertilizers
enrich local waters. This eutrophication may be beneficial in some cases, but
often the amount of nutrients delivered leads to excessive plankton blooms
that eventually decay and cause oxygen depletion. Pathogenic organisms in
human wastes, like the cholera virus, can be filtered out of water near
sewage outfalls by mussels and clams, and then be transmitted to humans
who consume this seafood.

10 Radioactive wastes do not presently occur in concentrations that
threaten marine Ufe, although it is known that some organisms, particularly
seaweeds and bivalves, can accumulate radionuclides from waters around
nuclear plants.

11 Power plants release heated water that elevates ambient seawater
temperature and thereby affeets marine communities within the immediate
area. In some cases the heated effluent is used to enhance growth rates of
organisms grown in culture, but often the community changes are
detrimental or unwanted.

12 Some marine communities and ecosystems have been changed through
the delibérate or accidental transplantation of species. Increased commercial
shipping has accelerated the rate of introduction of species into new
environments, with an estimated 3000 species of marine plants and animáis
being carried daily across oceans in the ballast tanks of ships. Many do not
survive but some, like the ctenophore Mnemiopsis or the zebra mussel, have
major impaets on their new environments.

13 Estuaries and mangrove swamps are productive Coastal ecosystems that
constitute important spawning and nursery grounds for many fish, harbour
shellfish populations, and provide rich feeding grounds for birds. As well,
mangrove swamps buffer coastlines from erosión and inundation during
tropical storms. However, these ecosystems are often heavily affected by
human activities such as land reclamation, disposal of sewage and industrial
wastes, and eutrophication.

265

14 Coral reefs are declining throughout the world. Expanding human
populations near reefs and increasing tourism have accelerated development
and have brought growing pressure to exploit reef resources. Coral reefs are
detrimentally affected by increased sedimentation resulting from land
development and subsequent erosión, and from eutrophication stemming
from sewage disposal and agricultural runoff. In some locales, the coral is
mined as building material. Destructive fishing techniques remove large
numbers of fish, change species composition on the reef, and damage coráis
directly. Coráis world-wide have been affected by elevations in seawater
temperature, and global warming is a potential danger to reef communities.

Now try the following questions to consolídate your understanding of this
Chapter.

QUESTION 9.5 The subject of human impacts on marine bioia is much too
large to cover fully in this chapter. What other impacts can you ihink of that
might have been included?

QUESTION 9.6 Is ti possi ble to harvest fish from the sea without causing
changes in marine ecosystems?

QUESTION 9.7 Does sewage disposal al sea have any beneficia! impact?

QUESTION 9.8 Estuary A rece i ves freshwaier with a htgh sediment load,
whereas Estuary B receives retalively clear river water. Both receive
approximately equal amounts of urban pollutants. Which estuar> r would have
lower concentrations of these pollutants in the water, and which would have
lower polluiani concentrations in the sediments?

APPENDIX 1

GEOLOGIC TIME SCALE

Approx. time unit began

(millions of years ago)

Eras

Periods

Epochs

Major biotic events

0.01

Quaternary

Holocene

Modern man appears

Pleistocene

Early humans appear (2-3 mya)

Cenzoic

Pliocene

Miocene

Tertiary

Oligocene

Early anthropoids appear (40 mya)
Heteropods appear

Eocene

Marine mammals, shelled pteropods appear
(55 mya)

Seabirds appear (60 mya)

Paleocene

Dinosaurs become extinct (65 mya)

Diatoms and silicoflagellates appear

(100 mya)

140

Cretaceous

Ammonoid cephalopods become extinct
Coccolithophorids and forams appear;

number of dinoflagellate species

increases

200

Mesozoic

Jurassic

Marine reptiles and primitive birds appear
(200 mya)

Most nautiloid cephalopods go extinct

240

Triassic

Dinosaurs and mammals appear; most

nautiloid cephalopods become extinct

290

Permian

Trilobites become extinct

Teleost fishes appear (300 mya)

360

Carboniferous

410

Paleozoic

Devonian

Land plants appear (420 mya)

435

Silurian

Dinoflagellates, barnacles appear;

bivalves become abundant
Elasmobranchs (sharks), ammonoid

cephalopods appear (400-450 mya)

500

Ordovician

Tintinnids evolve, coral reefs form
Primitive fish appear (550 mya)

570

Cambrian

Trilobites dominate; ostracods, echino-

derms, nautiloid cephalopods appear
Marine algae become highly diversified

Precambrian

Radiolaria appear (600 mya)

Abundant fossils of marine invertebrates

(e.g. jellyfish, sponges, molluscs)
Oldest fossils of shelled marine animáis

(650 mya)

First multicellular seaweeds (800 mya)

1000

Calcareous algae (1500 mya)

2000

Multicellular life appears (2100-1900 mya)
Earliest photosynthetic organisms

(2800-2500 mya)

3000

Oldest fossils of cyanobacteria (3500 mya)
Life originates (3900-3500 mya)

4000

Atmosphere and ocean form (4400 mya)

5000

Earth’s crust forms (4600 mya)

APPENDIX 2 ■ CONVERSIONS

UNITS OF AREA

1 squarecentimetre(cm 2 )

= 100 mm 2 = 0.155 square inch

1 square metre (m 2 )

= 10 4 cm 2 =10.8 square feet

1 square kilometre (km 2 )

= 10 6 m 2 = 247.1 acres

1 hectare (ha)

= 10000 m 2

UNITS OF CONCENTRATION

molar concentration (M)

= gramme molecular weight per litre

¡ig litre -1

mg m -3

parts per million (ppm)

= mg litre -1

parts per billion (ppb)

= /xg litre -1

parts per trillion (ppt)

= 10 -3 /xg litre -1 = 1 nanogramme litre -

/xg litre 1 -r molecular weight = /xM = /xmol litre

UNITS OF LENGTH

1 ángstrom (Á)

= 0.0001 micron

1 nanometre (nm)

= 10 -9 metres

1 micron (/x)

= 0.001 millimetre (or 10 -3 mm) = 10 -(

1 millimetre (mm)

= 1000 microns = 0.001 metre

1 centimetre (cm)

= 10 millimetres = 0.394 inch

1 decimetre (dm)

= 0.1 metre

1 metre (m)

= 100 centimetres = 3.28 feet

1 kilometre (km)

= 1000 metres = 3280 feet

1 kilometre (km)

= 0.62 statute mile = 0.54 nautical mile

1 inch (in)

= 2.54 centimetres

1 foot (ft)

= 0.3048 metre

1 yard (yd)

= 3 feet = 0.91 metre

1 fathom

= 6 feet =1.83 metres

1 statute mile

= 1.6 kilometres = 0.87 nautical mile

1 nautical mile

= 1.85 kilometres =1.15 statute miles

UNITS OF MASS

1 milligram (mg)

= 0.001 gramme

1 kilogram (kg)

= 1000 grammes = 2.2 pounds

1 tonne (t)

= 1 metric ton = 10 6 grammes

1 pound (Ib)

— 453.6 grammes

UNITS OF TIME

1 minute (min)

= 60 seconds (s)

1 hour (h)

= 3600 s

1 day (d)

= 86400 s

1 year (yr)

= 365 d

268

UNITS OF VELOCITY

1 kilometre per hour = 27.8 centimetres per second
1 knot (kn) = 1 nautical mile per hour = 51.5 centimetres per second

UNITS OF VOLUME

1 millilitre (mi) = 0.001 litre = 1 cm 3 (or 1 cc)

1 litre (1) = 1000 cm 3 = 10" 3 m

1 cubic metre (m 3 ) = 1000 litres

UNITS USED TO MEASURE SOLAR RADIATION ENERGY

1 einstein (E) = 6.02 x 10 23 photons = 1 mole of photons
1 watt m" 2 ^ 4.16 ± 0.42 ¡i einsteins m -2 s _1
(The above relationship applies only to PAR)

1 joule m~ 2 s _1 = 1 watt m -2
1 calorie cm -2 min _l ~ 700 watts irf 2
1 langley = 1 calorie cm -2

UNITS USED IN PR00UCTI0N STUDIES

1 calorie (cal) = 4.184 joules (J)

1 kilocalorie (kcal) = 1000 cal
1 gramme carbón (g C) ^ 10 kcal

1 gC^2 g ash-free dry weight (where ash-free dry weight is dry weight
less the weight of inorganic components such as shells)

1 g ash-free dry weight ^ 21 kJ
1 g organic C «s 42 kJ
1 g ash-free dry weight ^ 5 g wet weight
1 litre O 2 = 4.825 kcal
1 g carbohydrate ^ 4.1 kcal
1 g protein ^5.65 kcal
1 g fat ^ 9.45 kcal

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